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title: Background - The Falikov Kimball 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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<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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<div id="page-header">
<p>2 Background</p>
<hr />
</div>
<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>Here we will only discuss the hypercubic lattices, i.e the chain, the
square lattice, the cubic lattice and so on. 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>. The ground
state phenomenology as the model is doped away from the half-filled
state can be rich <span class="citation"
data-cites="jedrzejewskiFalicovKimballModels2001 gruberGroundStatesSpinless1990"> [<a
href="#ref-jedrzejewskiFalicovKimballModels2001"
role="doc-biblioref">7</a>,<a href="#ref-gruberGroundStatesSpinless1990"
role="doc-biblioref">8</a>]</span> but from this point we will only
consider the half-filled point.</p>
<p>At half-filling and on bipartite lattices, FK the model is
particle-hole (PH) 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">9</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>. See appendix <a
href="../6_Appendices/A.1_Particle_Hole_Symmetry.html#particle-hole-symmetry">A.1</a>
for a full derivation of the PH symmetry.</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 at which point 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> which I will refer to as the
spins.</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 exactly 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">10</a><a
href="#ref-herrmannNonequilibriumDynamicalCluster2016"
role="doc-biblioref">13</a>]</span>.</p>
</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 diagram of the Falikov-Kimball model in dimensions greater than two. At low temperature the classical fermions (spins) settle into an ordered charge density wave state (antiferromagnetic state). The schematic diagram for the Hubbard model is the same. Reproduced from  [10,14]" />
<figcaption aria-hidden="true"><span>Figure 2:</span> Schematic Phase
diagram of the Falikov-Kimball model in dimensions greater than two. At
low temperature the classical fermions (spins) settle into an ordered
charge density wave state (antiferromagnetic state). The schematic
diagram for the Hubbard model is the same. Reproduced from <span
class="citation"
data-cites="antipovInteractionTunedAndersonMott2016 antipovCriticalExponentsStrongly2014"> [<a
href="#ref-antipovCriticalExponentsStrongly2014"
role="doc-biblioref">10</a>,<a
href="#ref-antipovInteractionTunedAndersonMott2016"
role="doc-biblioref">14</a>]</span></figcaption>
</figure>
</div>
<p>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 ordered phase <span class="citation"
data-cites="maskaThermodynamicsTwodimensionalFalicovKimball2006"> [<a
href="#ref-maskaThermodynamicsTwodimensionalFalicovKimball2006"
role="doc-biblioref">15</a>]</span>. In terms of the immobile electrons
this corresponds to them occupying only one of the two sublattices A and
B this is known as a charge density wave (CDW) phase. In terms of spins
this is an AFM phase.</p>
<p>In the disordered region above <span
class="math inline">\(T_c(U)\)</span> there are two insulating phases.
For weak interactions <span class="math inline">\(U &lt;&lt; t\)</span>,
thermal fluctuations in the spins act as an effective disorder potential
for the fermions, causing them to localise and giving rise to an
Anderson insulating state <span class="citation"
data-cites="andersonAbsenceDiffusionCertain1958"> [<a
href="#ref-andersonAbsenceDiffusionCertain1958"
role="doc-biblioref">16</a>]</span> which we will discuss more in
section <a
href="../2_Background/2.3_Disorder.html#bg-disorder-and-localisation">2.3</a>.
For strong interactions <span class="math inline">\(U &gt;&gt;
t\)</span>, the spins are not ordered but nevertheless their interaction
with the electrons opens a gap, leading a Mott insulator analogous to
that of the Hubbard model <span class="citation"
data-cites="brandtThermodynamicsCorrelationFunctions1989"> [<a
href="#ref-brandtThermodynamicsCorrelationFunctions1989"
role="doc-biblioref">17</a>]</span>.</p>
<p>By contrast, in the one dimensional FK model there is no
finite-temperature phase transition (FTPT) to an ordered CDW phase <span
class="citation" data-cites="liebAbsenceMottTransition1968"> [<a
href="#ref-liebAbsenceMottTransition1968"
role="doc-biblioref">18</a>]</span>. Indeed dimensionality is crucial
for the physics of both localisation and FTPTs. In one dimension,
disorder generally dominates: even the weakest disorder exponentially
localises <em>all</em> single particle eigenstates. Only longer-range
correlations of the disorder potential can potentially induce
localisation-delocalisation transitions in one dimension <span
class="citation"
data-cites="aubryAnalyticityBreakingAnderson1980 dassarmaLocalizationMobilityEdges1990 dunlapAbsenceLocalizationRandomdimer1990"> [<a
href="#ref-aubryAnalyticityBreakingAnderson1980"
role="doc-biblioref">19</a><a
href="#ref-dunlapAbsenceLocalizationRandomdimer1990"
role="doc-biblioref">21</a>]</span>. Thermodynamically, short-range
interactions cannot overcome thermal defects in one dimension which
prevents ordered phases at non-zero temperature <span class="citation"
data-cites="goldshteinPurePointSpectrum1977 abrahamsScalingTheoryLocalization1979 kramerLocalizationTheoryExperiment1993"> [<a
href="#ref-goldshteinPurePointSpectrum1977"
role="doc-biblioref">22</a><a
href="#ref-kramerLocalizationTheoryExperiment1993"
role="doc-biblioref">24</a>]</span>.</p>
<p>However, the absence of an FTPT in the short ranged FK chain is far
from obvious because the Ruderman-Kittel-Kasuya-Yosida (RKKY)
interaction mediated by the fermions <span class="citation"
data-cites="kasuyaTheoryMetallicFerro1956 rudermanIndirectExchangeCoupling1954 vanvleckNoteInteractionsSpins1962 yosidaMagneticPropertiesCuMn1957"> [<a
href="#ref-kasuyaTheoryMetallicFerro1956" role="doc-biblioref">25</a><a
href="#ref-yosidaMagneticPropertiesCuMn1957"
role="doc-biblioref">28</a>]</span> decays as <span
class="math inline">\(r^{-1}\)</span> in one dimension <span
class="citation" data-cites="rusinCalculationRKKYRange2017"> [<a
href="#ref-rusinCalculationRKKYRange2017"
role="doc-biblioref">29</a>]</span>. This could in principle induce the
necessary long-range interactions for the classical Ising background to
order at low temperatures <span class="citation"
data-cites="thoulessLongRangeOrderOneDimensional1969 peierlsIsingModelFerromagnetism1936"> [<a
href="#ref-thoulessLongRangeOrderOneDimensional1969"
role="doc-biblioref">30</a>,<a
href="#ref-peierlsIsingModelFerromagnetism1936"
role="doc-biblioref">31</a>]</span>. However, Kennedy and Lieb
established rigorously that at half-filling a CDW phase only exists at
<span class="math inline">\(T = 0\)</span> for the one dimensional FK
model <span class="citation"
data-cites="kennedyItinerantElectronModel1986"> [<a
href="#ref-kennedyItinerantElectronModel1986"
role="doc-biblioref">32</a>]</span>.</p>
<p>Based on this primacy of dimensionality, we will go digging into the
one dimensional case. In chapter <a
href="../3_Long_Range_Falikov_Kimball/3.1_LRFK_Model.html#fk-model">3</a>
we will construct a generalised one-dimensional FK model with long-range
interactions which induces the otherwise forbidden CDW phase at non-zero
temperature. To do this we will draw on theory of the Long Range Ising
Model which is the subject of the next section.</p>
</section>
<section id="long-ranged-ising-model" class="level2">
<h2>Long Ranged Ising model</h2>
<p>The suppression of phase transitions is a common phenomena in one
dimensional systems and the Ising model serves as a great illustration
of this. In terms of classical spins <span class="math inline">\(S_i =
\pm \frac{1}{2}\)</span> the standard Ising model reads</p>
<p><span class="math display">\[H_{\mathrm{I}} = \sum_{\langle ij
\rangle} S_i S_j\]</span></p>
<p>Like the FK model, the Ising model shows an FTPT to an ordered state
only in two dimensions and above. This can be understood via Peierls
argument <span class="citation"
data-cites="peierlsIsingModelFerromagnetism1936 kennedyItinerantElectronModel1986"> [<a
href="#ref-peierlsIsingModelFerromagnetism1936"
role="doc-biblioref">31</a>,<a
href="#ref-kennedyItinerantElectronModel1986"
role="doc-biblioref">32</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. If this value is negative it implies that the
ordered state is unstable with respect to domain wall defects, and they
will thus proliferate, destroying the ordered phase. If we consider the
scaling of the two terms with system size <span
class="math inline">\(L\)</span> we see that short range interactions
produce a constant energy penalty <span class="math inline">\(\Delta
E\)</span> for a domain wall. In contrast, the number of such single
domain wall states scales linearly with system size 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> which is why they can support
ordered phases. This argument does not quite apply to the FK model
because of the aforementioned RKKY interaction. Instead this argument
will give us insight into how to recover an ordered phase in the one
dimensional FK model.</p>
<p>In contrast the long range Ising (LRI) model <span
class="math inline">\(H_{\mathrm{LRI}}\)</span> can have an FTPT in one
dimension.</p>
<p><span class="math display">\[H_{\mathrm{LRI}} = \sum_{ij} J(|i-j|)
S_i S_j = J \sum_{i\neq j} |i - j|^{-\alpha} S_i S_j\]</span></p>
<p>Renormalisation group analyses show that the LRI model has an ordered
phase in 1D for <span class="math inline">\(1 &lt; \alpha &lt;
2\)</span>  <span class="citation"
data-cites="dysonExistencePhasetransitionOnedimensional1969"> [<a
href="#ref-dysonExistencePhasetransitionOnedimensional1969"
role="doc-biblioref">33</a>]</span>. Peierls argument can be
extended <span class="citation"
data-cites="thoulessLongRangeOrderOneDimensional1969"> [<a
href="#ref-thoulessLongRangeOrderOneDimensional1969"
role="doc-biblioref">30</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">\(|\ldots\uparrow\uparrow\uparrow\uparrow\ldots\rangle\)</span>
and a domain wall state <span
class="math inline">\(|\ldots\uparrow\uparrow\downarrow\downarrow\ldots\rangle\)</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. The behaviour then depends crucially on the sum scales with
system size. 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 to a constant in the thermodynamic
limit then the free energy is analytic <span class="citation"
data-cites="ruelleStatisticalMechanicsOnedimensional1968"> [<a
href="#ref-ruelleStatisticalMechanicsOnedimensional1968"
role="doc-biblioref">34</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">33</a>]</span>.</p>
<p>With a power law form for <span class="math inline">\(J(n)\)</span>,
there are a few cases to consider:</p>
<p>For <span class="math inline">\(\alpha = 0\)</span> i.e infinite
range interactions, the Ising model is exactly solvable and mean field
theory is exact <span class="citation"
data-cites="lipkinValidityManybodyApproximation1965"> [<a
href="#ref-lipkinValidityManybodyApproximation1965"
role="doc-biblioref">35</a>]</span>. This limit is the same as the
infinite dimensional limit.</p>
<p>For <span class="math inline">\(\alpha \leq 1\)</span> we have very
slowly decaying interactions. <span class="math inline">\(\Delta
E\)</span> does not converge as a function of system size so the
Hamiltonian is non-extensive, a topic not without some considerable
controversy <span class="citation"
data-cites="grossNonextensiveHamiltonianSystems2002 lutskoQuestioningValidityNonextensive2011 wangCommentNonextensiveHamiltonian2003"> [<a
href="#ref-grossNonextensiveHamiltonianSystems2002"
role="doc-biblioref">36</a><a
href="#ref-wangCommentNonextensiveHamiltonian2003"
role="doc-biblioref">38</a>]</span> that we will not consider further
here.</p>
<p>For <span class="math inline">\(1 &lt; \alpha &lt; 2\)</span>, we get
a phase transition to an ordered state at a finite temperature, this is
what we want!</p>
<p>For <span class="math inline">\(\alpha = 2\)</span>, 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">30</a>]</span>.</p>
<p>Finally, for <span class="math inline">\(2 &lt; \alpha\)</span> we
have very quickly decaying interactions and domain walls again have a
finite energy penalty, hence Peirels argument holds and there is no
phase transition.</p>
<p>One final complexity is that for <span
class="math inline">\(\tfrac{3}{2} &lt; \alpha &lt; 2\)</span>
renormalisation group methods show that the critical point has
non-universal critical exponents that depend on <span
class="math inline">\(\alpha\)</span>  <span class="citation"
data-cites="fisherCriticalExponentsLongRange1972"> [<a
href="#ref-fisherCriticalExponentsLongRange1972"
role="doc-biblioref">39</a>]</span>. To avoid this potential confounding
factors we will park ourselves at <span class="math inline">\(\alpha =
1.25\)</span> when we apply these ideas to the FK model.</p>
<p>Were we to extend this to arbitrary dimension <span
class="math inline">\(d\)</span> we would find that thermodynamics
properties generally both <span class="math inline">\(d\)</span> and
<span class="math inline">\(\alpha\)</span>, long range interactions can
modify the effective dimension of thermodynamic systems <span
class="citation"
data-cites="angeliniRelationsShortrangeLongrange2014"> [<a
href="#ref-angeliniRelationsShortrangeLongrange2014"
role="doc-biblioref">40</a>]</span>.</p>
<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: The thermodynamic behaviour of the long range Ising model H_{\mathrm{LRI}} = J \sum_{i\neq j} |i - j|^{-\alpha} S_i S_j as the exponent of the interaction \alpha is varied." />
<figcaption aria-hidden="true"><span>Figure 3:</span> The thermodynamic
behaviour of the long range Ising model <span
class="math inline">\(H_{\mathrm{LRI}} = J \sum_{i\neq j} |i -
j|^{-\alpha} S_i S_j\)</span> as the exponent of the interaction <span
class="math inline">\(\alpha\)</span> is varied.</figcaption>
</figure>
</div>
<p>Next Section: <a
href="../2_Background/2.2_HKM_Model.html#the-kitaev-honeycomb-model">The
Kitaev Honeycomb Model</a></p>
</section>
</section>
<section id="bibliography" class="level1 unnumbered">
<h1 class="unnumbered">Bibliography</h1>
<div id="refs" class="references csl-bib-body" role="doc-bibliography">
<div id="ref-hubbardj.ElectronCorrelationsNarrow1963" class="csl-entry"
role="doc-biblioentry">
<div class="csl-left-margin">[1] </div><div
class="csl-right-inline">Hubbard, J., <em><a
href="https://doi.org/10.1098/rspa.1963.0204">Electron Correlations in
Narrow Energy Bands</a></em>, Proceedings of the Royal Society of
London. Series A. Mathematical and Physical Sciences
<strong>276</strong>, 238 (1963).</div>
</div>
<div id="ref-falicovSimpleModelSemiconductorMetal1969" class="csl-entry"
role="doc-biblioentry">
<div class="csl-left-margin">[2] </div><div class="csl-right-inline">L.
M. Falicov and J. C. Kimball, <em><a
href="https://doi.org/10.1103/PhysRevLett.22.997">Simple Model for
Semiconductor-Metal Transitions: Sm${\mathrm{B}}_{6}$ and
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