{"id":2,"date":"2018-06-10T21:05:25","date_gmt":"2018-06-10T21:05:25","guid":{"rendered":"http:\/\/localhost\/wordpress\/?page_id=2"},"modified":"2021-01-08T23:23:41","modified_gmt":"2021-01-08T23:23:41","slug":"research","status":"publish","type":"page","link":"http:\/\/localhost\/wordpress\/research\/","title":{"rendered":"Research"},"content":{"rendered":"<p>Here you can find a snapshot of our current interests and projects. They all relate in some way to topological materials, broadly defined as those materials where a topological invariant enforces the existence of low energy electronic quasiparticles which cannot be removed form the system by perturbing it. These quasiparticle excitations often lead to exotic physical properties, and a common theme in our work is to find these properties in old and new materials, and to figure out how to access them in experiments.<\/p>\n<ul>\n<li><strong>Topological nodal semimetals<\/strong> &#8211; The energy band arrangement in a material determines whether it is insulating or metallic. A special situation arises when the conduction and the valence band cross at a single isolated point in momentum space, a nodal degeneracy which is topologically protected because it is a source or sink of Berry flux. The simplest of these crossings, known as a Weyl node because of its similarity with the Weyl fermion, has been recently observed in TaAs with ARPES. We have discovered <a href=\"https:\/\/www.nature.com\/articles\/ncomms15995\">an unexpected example of topological quantization<\/a> in these systems: the growth of photocurrent generated by circularly polarized light is exactly quantized in terms of fundamental constants because it measures directly the monopole charge of the Weyl node. This effect requires a Weyl semimetal in a chiral lattice structure (one without orientation reversing symmetries), but no proper material that realizes it has been found. However, we have also shown that this <a href=\"https:\/\/arxiv.org\/abs\/1806.09642\">effect is preserved for generalizations of Weyl nodes<\/a> with spins larger than 1\/2 and multiband touchings, which have indeed been found experimentally in CoSi and related materials (as in our last <a href=\"https:\/\/science.sciencemag.org\/content\/369\/6500\/179.abstract\">ARPES collaboration<\/a>). These materials are currently the best candidates to observe the quantized CPGE, and in a recent collaboration we have indeed shown that <a href=\"https:\/\/www.nature.com\/articles\/s41467-020-20408-5\">the measured CPGE is consistent with predictions<\/a> and that the quantized regime should be in principle accessible.<\/li>\n<\/ul>\n<h3>Less recent topics<\/h3>\n<ul>\n<li><strong>Topological insulators<\/strong> &#8211; Probably the most known example of topological materials, these are bulk non-magnetic insulators with strong spin-orbit coupling which feature protected Dirac fermion surface states, and are realized in compounds of the Bi2Se3 family. <a href=\"https:\/\/doi.org\/10.1103\/PhysRevLett.115.096802\">One of my recent predictions<\/a> is that due to the spin-momentum locking of Dirac fermions, a p-n junction of this material in a magnetic field realizes a perfect Mach-Zehnder interferometer, with unique conductance modulations that can be observed in current transport setups. Topological insulators also come in a two-dimensional variety, found for example in HgTe quantum wells, which is again bulk insulating but presents helical edge states. <a href=\"https:\/\/doi.org\/10.1103\/PhysRevLett.119.226401\">In a recent experimental collaboration<\/a>, we have shown how magnetic fields in similar junctions allow to generate networks of helical and chiral transport channels, which generate Fabry-Perot interference patterns in the conductance which reflect the existence of such networks.<\/li>\n<li><strong>Topological superconductors<\/strong> &#8211; Superconductors can be topological in a sense analogous to insulators: their Bogoliubov-de Gennes quasiparticle spectrum is gapped, but they can have surface states that are topologically protected. Topological superconductors exist both in bulk form (3D), as films or interfaces (2D) and as wires (1D), and each class has particular boundary modes, which are always described by some incarnation of the Majorana equation. \u00a0The bulk form is proposed to be realized in doped Bi2Se3, but different dopants appear to display different phenomenology. The Nb-doped version is particularly intriguing as it appears to be magnetic, and <a href=\"https:\/\/doi.org\/10.1103\/PhysRevB.95.201110\">we have put forward a theory<\/a> in terms of a multicomponent order parameter that matches the current observations, and predicts the possibility of bulk nodes in the superconducting gap.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Here you can find a snapshot of our current interests and projects. They all relate in some way to topological materials, broadly defined as those materials where a topological invariant <a class=\"more-link\" href=\"http:\/\/localhost\/wordpress\/research\/\">Continue Reading &rarr;<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"open","template":"","meta":[],"_links":{"self":[{"href":"http:\/\/localhost\/wordpress\/wp-json\/wp\/v2\/pages\/2"}],"collection":[{"href":"http:\/\/localhost\/wordpress\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/localhost\/wordpress\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/localhost\/wordpress\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/localhost\/wordpress\/wp-json\/wp\/v2\/comments?post=2"}],"version-history":[{"count":10,"href":"http:\/\/localhost\/wordpress\/wp-json\/wp\/v2\/pages\/2\/revisions"}],"predecessor-version":[{"id":272,"href":"http:\/\/localhost\/wordpress\/wp-json\/wp\/v2\/pages\/2\/revisions\/272"}],"wp:attachment":[{"href":"http:\/\/localhost\/wordpress\/wp-json\/wp\/v2\/media?parent=2"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}