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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Pi Josephson junction</span></span>
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<p><br>
A <a href="Josephson_junction" class="mw-redirect" title="Josephson junction">Josephson junction</a> (JJ) is a quantum mechanical device which is made of two superconducting electrodes separated by a barrier (thin insulating tunnel barrier, normal metal, semiconductor, ferromagnet, etc.).
A <b><span class="texhtml mvar" style="font-style:italic;">π</span> Josephson junction</b> is a Josephson junction in which the <a href="Josephson_phase" class="mw-redirect" title="Josephson phase">Josephson phase</a> <i>φ</i> equals <span class="texhtml mvar" style="font-style:italic;">π</span> in the ground state, i.e. when no external current or <a href="Magnetic_field" title="Magnetic field">magnetic field</a> is applied.
</p>
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<div class="mw-heading mw-heading2"><h2 id="Background">Background</h2></div>
<p>The <a href="Supercurrent" title="Supercurrent">supercurrent</a> <i>I</i><sub><i>s</i></sub> through a Josephson junction is generally given by <i>I</i><sub><i>s</i></sub> = <i>I</i><sub><i>c</i></sub>sin(<i>φ</i>),
where φ is the phase difference of the superconducting wave functions of the two
electrodes, i.e. the Josephson phase.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>
The critical current <i>I</i><sub><i>c</i></sub> is the maximum supercurrent that can exist through the Josephson junction.
In experiment, one usually causes some current through the Josephson junction and the junction reacts by changing the Josephson phase. From the above formula it is clear that the phase <i>φ</i> = arcsin(<i>I</i>/<i>I</i><sub><i>c</i></sub>), where <i>I</i> is the applied (super)current.
</p><p>Since the phase is 2<span class="texhtml mvar" style="font-style:italic;">π</span>-periodic, i.e. <i>φ</i> and <i>φ</i> + 2<span class="texhtml mvar" style="font-style:italic;">π</span><i>n</i> are physically equivalent, without losing generality, the discussion below refers to the interval 0 ≤ <i>φ</i> < 2<span class="texhtml mvar" style="font-style:italic;">π</span>.
</p><p>When no current (<i>I</i> = 0) exists through the Josephson junction, e.g. when the junction is disconnected, the junction is in the ground state and the Josephson phase across it is zero (<i>φ</i> = 0). The phase can also be <i>φ</i> = <span class="texhtml mvar" style="font-style:italic;">π</span>, also resulting in no current through the junction. It turns out that the state with <i>φ</i> = <span class="texhtml mvar" style="font-style:italic;">π</span> is <i>unstable</i> and corresponds to the <a href="Josephson_energy" class="mw-redirect" title="Josephson energy">Josephson energy</a> maximum, while the state <i>φ</i> = 0 corresponds to the Josephson energy minimum and <i>is</i> a ground state.
</p><p>In certain cases, one may obtain a Josephson junction where the critical current is negative (<i>I</i><sub><i>c</i></sub> < 0). In this case, the first Josephson relation becomes
</p>
<dl><dd><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle I_{s}=-|I_{c}|\sin(\varphi )=|I_{c}|\sin(\varphi +\pi )}">
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<annotation encoding="application/x-tex">{\displaystyle I_{s}=-|I_{c}|\sin(\varphi )=|I_{c}|\sin(\varphi +\pi )}</annotation>
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</math></span><img src="./f57b347d6cf6f1c6169717560859964414f4120d.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:33.87ex; height:2.843ex;" alt="{\displaystyle I_{s}=-|I_{c}|\sin(\varphi )=|I_{c}|\sin(\varphi +\pi )}" loading="lazy"></span></dd></dl>
<p>The ground state of such a Josephson junction is <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \phi =\pi }">
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<mi>ϕ<!-- ϕ --></mi>
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<annotation encoding="application/x-tex">{\displaystyle \phi =\pi }</annotation>
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</math></span><img src="./a78f50e4da76006867400dbb8ad8e6747f55c048.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:5.816ex; height:2.509ex;" alt="{\displaystyle \phi =\pi }" loading="lazy"></span> and corresponds to the Josephson energy minimum, while the conventional state φ = 0 is unstable and corresponds to the Josephson energy maximum. Such a Josephson junction with <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \phi =\pi }">
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<annotation encoding="application/x-tex">{\displaystyle \phi =\pi }</annotation>
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</math></span><img src="./a78f50e4da76006867400dbb8ad8e6747f55c048.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:5.816ex; height:2.509ex;" alt="{\displaystyle \phi =\pi }" loading="lazy"></span> in the ground state is called a <span class="texhtml mvar" style="font-style:italic;">π</span> Josephson junction.
</p><p><span class="texhtml mvar" style="font-style:italic;">π</span> Josephson junctions have quite unusual properties. For example, if one connects (shorts) the superconducting electrodes with the inductance <i>L</i> (e.g. <a href="Superconducting_wire" title="Superconducting wire">superconducting wire</a>), one may expect the spontaneous supercurrent circulating in the loop, passing through the junction and through inductance clockwise or counterclockwise. This supercurrent is spontaneous and belongs to the ground state of the system. The direction of its circulation is chosen at random. This supercurrent will of course induce a magnetic field which can be detected experimentally. The magnetic flux passing through the loop will have the value from 0 to a half of <a href="Magnetic_flux_quanta" class="mw-redirect" title="Magnetic flux quanta">magnetic flux quanta</a>, i.e. from 0 to Φ<sub>0</sub>/2, depending on the value of inductance <i>L</i>.
</p>
<div class="mw-heading mw-heading2"><h2 id="Technologies_and_physical_principles">Technologies and physical principles</h2></div>
<ul><li><b>Ferromagnetic Josephson junctions</b>. Consider a Josephson junction with a ferromagnetic Josephson barrier, i.e. the multilayers superconductor-ferromagnet-superconductor (SFS) or superconductor-insulator-ferromagnet-superconductor (SIFS). In such structures the superconducting order parameter inside the F-layer oscillates in the direction perpendicular to the junction plane. As a result, for certain thicknesses of the F-layer and temperatures, the order parameter may become +1 at one superconducting electrode and −1 at the other superconducting electrode. In this situation one gets a <span class="texhtml mvar" style="font-style:italic;">π</span> Josephson junction. Note that inside the F-layer the competition of different solutions takes place and the one with the lower energy wins out. Various ferromagnetic <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \pi }">
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<annotation encoding="application/x-tex">{\displaystyle \pi }</annotation>
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</math></span><img src="./9be4ba0bb8df3af72e90a0535fabcc17431e540a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.332ex; height:1.676ex;" alt="{\displaystyle \pi }" loading="lazy"></span> junctions have been fabricated: SFS junctions with weak ferromagnetic interlayers;<sup id="cite_ref-Ryazanov:SFS_2-0" class="reference"><a href="#cite_note-Ryazanov:SFS-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> SFS junctions with strong ferromagnetic interlayers, such as Co, Ni,<sup id="cite_ref-Bannykh:SIFS_3-0" class="reference"><a href="#cite_note-Bannykh:SIFS-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> PdFe <sup id="cite_ref-Bolg_4-0" class="reference"><a href="#cite_note-Bolg-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> and NiFe<sup id="cite_ref-Robinson:SFS_5-0" class="reference"><a href="#cite_note-Robinson:SFS-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> SIFS junctions;<sup id="cite_ref-Bannykh:SIFS_3-1" class="reference"><a href="#cite_note-Bannykh:SIFS-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Kontos:SIFS_6-0" class="reference"><a href="#cite_note-Kontos:SIFS-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Lark_7-0" class="reference"><a href="#cite_note-Lark-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Weides:SIFS_8-0" class="reference"><a href="#cite_note-Weides:SIFS-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> and S-Fi-S junctions.<sup id="cite_ref-Vavra:SFiS_9-0" class="reference"><a href="#cite_note-Vavra:SFiS-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup></li></ul>
<ul><li><b>Josephson junctions with unconventional order parameter symmetry</b>. Novel superconductors, notably high temperature cuprate superconductors, have an anisotropic <a href="Ginzburg-Landau_Theory" class="mw-redirect" title="Ginzburg-Landau Theory">superconducting order parameter</a> which can change its sign depending on the direction. In particular, a so-called d-wave order parameter has a value of +1 if one looks along the crystal axis <i>a</i> and −1 if one looks along the crystal axis <i>b</i>. If one looks along the <i>ab</i> direction (45° between <i>a</i> and <i>b</i>) the order parameter vanishes. By making Josephson junctions between d-wave superconducting films with different orientations or between d-wave and conventional isotropic s-wave superconductors, one can get a phase shift of <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \pi }">
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</math></span><img src="./9be4ba0bb8df3af72e90a0535fabcc17431e540a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.332ex; height:1.676ex;" alt="{\displaystyle \pi }" loading="lazy"></span>. Nowadays there are several realizations of <span class="texhtml mvar" style="font-style:italic;">π</span> Josephson junctions of this type:
<ul><li>tri-crystal <a href="Grain_boundary" title="Grain boundary">grain boundary</a> Josephson junctions,<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup></li>
<li>tetra-crystal grain boundary Josephson junctions,<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup></li>
<li>d-wave/s-wave ramp zigzag Josephson junctions,<sup id="cite_ref-r1_13-0" class="reference"><a href="#cite_note-r1-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup></li>
<li>tilt-twist grain boundary Josephson junctions,<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup></li>
<li>p-wave based Josephson junctions.</li></ul></li>
<li>Superconductor–normal metal–superconductor (SNS) Josephson junctions with non-equilibrium electron distribution in N-layer.<sup id="cite_ref-Beselmans:NonEqPiJJ_18-0" class="reference"><a href="#cite_note-Beselmans:NonEqPiJJ-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup></li>
<li>Superconductor–<a href="Quantum_dot" title="Quantum dot">quantum dot</a>–superconductor (S-QuDot-S) Josephson junctions (implemented by <a href="Carbon_nanotube" title="Carbon nanotube">carbon nanotube</a> Josephson junctions).<sup id="cite_ref-CNT-JJ_19-0" class="reference"><a href="#cite_note-CNT-JJ-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup></li></ul>
<div class="mw-heading mw-heading2"><h2 id="Historical_developments">Historical developments</h2></div>
<p>Theoretically, the first time the possibility of creating a <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \pi }">
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</math></span><img src="./9be4ba0bb8df3af72e90a0535fabcc17431e540a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.332ex; height:1.676ex;" alt="{\displaystyle \pi }" loading="lazy"></span> Josephson junction was discussed by Bulaevskii <i>et al.</i> ,
<sup id="cite_ref-Bulaevskii:pi-loop_20-0" class="reference"><a href="#cite_note-Bulaevskii:pi-loop-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> who considered a Josephson junction with <a href="Paramagnetism" title="Paramagnetism">paramagnetic</a> scattering in the barrier. Almost one decade later, the possibility of having a <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \pi }">
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<annotation encoding="application/x-tex">{\displaystyle \pi }</annotation>
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</math></span><img src="./9be4ba0bb8df3af72e90a0535fabcc17431e540a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.332ex; height:1.676ex;" alt="{\displaystyle \pi }" loading="lazy"></span> Josephson junction was discussed in the context of heavy fermion p-wave superconductors.
<sup id="cite_ref-Geshkenbein_21-0" class="reference"><a href="#cite_note-Geshkenbein-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> Experimentally, the first <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \pi }">
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<annotation encoding="application/x-tex">{\displaystyle \pi }</annotation>
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</math></span><img src="./9be4ba0bb8df3af72e90a0535fabcc17431e540a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.332ex; height:1.676ex;" alt="{\displaystyle \pi }" loading="lazy"></span> Josephson junction was a corner junction made of <a href="Yttrium_barium_copper_oxide" title="Yttrium barium copper oxide">yttrium barium copper oxide</a> (d-wave) and <a href="Lead" title="Lead">Pb</a> (s-wave) superconductors.<sup id="cite_ref-r1_13-1" class="reference"><a href="#cite_note-r1-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> The first unambiguous proof of a <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \pi }">
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<annotation encoding="application/x-tex">{\displaystyle \pi }</annotation>
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</math></span><img src="./9be4ba0bb8df3af72e90a0535fabcc17431e540a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.332ex; height:1.676ex;" alt="{\displaystyle \pi }" loading="lazy"></span> Josephson junction with a ferromagnetic barrier was given only a decade later.<sup id="cite_ref-Ryazanov:SFS_2-1" class="reference"><a href="#cite_note-Ryazanov:SFS-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> That work used a weak ferromagnet consisting of a copper-nickel alloy (Cu<sub><i>x</i></sub>Ni<sub>1−<i>x</i></sub>, with <i>x</i> around 0.5) and optimized it so that the <a href="Curie_temperature" title="Curie temperature">Curie temperature</a> was close to the superconducting transition temperature of the superconducting <a href="Niobium" title="Niobium">niobium</a> leads.
</p>
<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Josephson_effect" title="Josephson effect">Josephson effect</a></li>
<li><a href="Varphi_Josephson_junction" class="mw-redirect" title="Varphi Josephson junction"><i>φ</i> Josephson junction</a></li>
<li><a href="Semifluxon" title="Semifluxon">Semifluxon</a></li>
<li><a href="Fractional_vortices" title="Fractional vortices">Fractional vortices</a></li>
<li><a href="Brian_D._Josephson" class="mw-redirect" title="Brian D. Josephson">Brian D. Josephson</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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</math></span><img src="./eb8c17dbd0bfd8ba385041a926df120ef31847c5.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.171ex; width:10.877ex; height:3.009ex;" alt="{\displaystyle s+id_{x^{2}-y^{2}}}" loading="lazy"></span> order-parameter symmetries". <i><a href="Annalen_der_Physik" title="Annalen der Physik">Annalen der Physik</a></i>. <b>8</b> (6): 511. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1999AnP...511..511C">1999AnP...511..511C</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1002%2F%28SICI%291521-3889%28199909%298%3A6%3C511%3A%3AAID-ANDP511%3E3.0.CO%3B2-K">10.1002/(SICI)1521-3889(199909)8:6<511::AID-ANDP511>3.0.CO;2-K</a>. <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:121496740">121496740</a>.</cite></span>
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<li id="cite_note-12"><span class="mw-cite-backlink"><b><a href="#cite_ref-12">^</a></b></span> <span class="reference-text">
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<li id="cite_note-r1-13"><span class="mw-cite-backlink">^ <a href="#cite_ref-r1_13-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-r1_13-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text">
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<mi>d</mi>
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<mi>x</mi>
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<mn>2</mn>
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<mo>−<!-- − --></mo>
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<mn>2</mn>
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<annotation encoding="application/x-tex">{\displaystyle d_{x^{2}-y^{2}}}</annotation>
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