{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,28]],"date-time":"2026-10-28T12:35:09Z","timestamp":1793190909214,"version":"4.5.0"},"reference-count":54,"publisher":"American Chemical Society (ACS)","issue":"42","license":[{"start":{"date-parts":[[2026,10,12]],"date-time":"2026-10-12T00:00:00Z","timestamp":1791763200000},"content-version":"stm-asf","delay-in-days":0,"URL":"https:\/\/doi.org\/10.15223\/policy-029"},{"start":{"date-parts":[[2026,10,12]],"date-time":"2026-10-12T00:00:00Z","timestamp":1791763200000},"content-version":"stm-asf","delay-in-days":0,"URL":"https:\/\/doi.org\/10.15223\/policy-037"},{"start":{"date-parts":[[2026,10,12]],"date-time":"2026-10-12T00:00:00Z","timestamp":1791763200000},"content-version":"stm-asf","delay-in-days":0,"URL":"https:\/\/doi.org\/10.15223\/policy-045"}],"funder":[{"DOI":"10.13039\/501100001659","name":"Deutsche Forschungsgemeinschaft","doi-asserted-by":"publisher","award":["SFB 1477 - 441234705"],"award-info":[{"award-number":["SFB 1477 - 441234705"]}],"id":[{"id":"10.13039\/501100001659","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"published-print":{"date-parts":[[2026,10,28]]},"abstract":"<jats:title>Abstract<\/jats:title>\n                  <jats:p>Hyperbolic media support high-k electromagnetic modes, enabling subdiffractional confinement, anomalous refraction, and enhanced light\u2013matter interactions. While most hyperbolic metamaterials rely on artificial structuring, natural hyperbolicity arises from crystal anisotropy and direction-dependent plasma frequencies. Here we provide direct experimental evidence for natural hyperbolicity in copper chalcogenides by spectroscopic ellipsometry in the visible\u2013near-infrared range. We demonstrate hyperbolic dispersion in bulk covellite CuS by identifying a spectral window where the real parts of the in-plane and out-of-plane permittivity components have opposite signs. The study is then expanded to size-controlled CuS nanosheets as oriented films. Using the Bruggeman effective medium approximation to account for the nanostructure\/void composite, we show that hyperbolicity persists upon nanoscaling and that the epsilon-near-zero condition and hyperbolic bandwidth shift with size. Finally, we demonstrate analogous behavior in klockmannite CuSe nanosheets, establishing copper chalcogenides as a tunable platform for natural hyperbolicity at the boundary of the NIR and visible ranges.<\/jats:p>","DOI":"10.1021\/acs.nanolett.6c02585","type":"journal-article","created":{"date-parts":[[2026,10,12]],"date-time":"2026-10-12T07:02:21Z","timestamp":1791788541000},"page":"14551-14559","source":"Crossref","is-referenced-by-count":0,"title":["Natural Hyperbolic Dispersion in Anisotropic Bulk\nand Nanoscaled Copper Chalcogenides"],"prefix":"10.1021","volume":"26","author":[{"given":"Urvi","family":"Parekh","sequence":"first","affiliation":[{"name":"University of Rostock , , Albert-Einstein-Stra\u00dfe 23 , ,","place":["Rostock, Germany, 18059"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Achyut","family":"Tiwari","sequence":"additional","affiliation":[{"name":"University of Stuttgart , , Pfaffenwaldring 57 , ,","place":["Stuttgart, Germany, 70569"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Bruno","family":"Gompf","sequence":"additional","affiliation":[{"name":"University of Stuttgart , , Pfaffenwaldring 57 , ,","place":["Stuttgart, Germany, 70569"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nadiia","family":"Didukh","sequence":"additional","affiliation":[{"name":"University of Rostock , , Albert-Einstein-Stra\u00dfe 23 , ,","place":["Rostock, Germany, 18059"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jivesh","family":"Kaushal","sequence":"additional","affiliation":[{"name":"University of Rostock , , Albert-Einstein-Stra\u00dfe 23 , ,","place":["Rostock, Germany, 18059"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Stefan","family":"Scheel","sequence":"additional","affiliation":[{"name":"University of Rostock , , Albert-Einstein-Stra\u00dfe 23 , ,","place":["Rostock, Germany, 18059"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8558-7389","authenticated-orcid":true,"given":"Christian","family":"Klinke","sequence":"additional","affiliation":[{"name":"University of Rostock , , Albert-Einstein-Stra\u00dfe 23 , ,","place":["Rostock, Germany, 18059"]},{"name":"University of Rostock , , Albert-Einstein-Stra\u00dfe 25 , ,","place":["Rostock, Germany, 18059"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8723-9222","authenticated-orcid":true,"given":"Rostyslav","family":"Lesyuk","sequence":"additional","affiliation":[{"name":"University of Rostock , , Albert-Einstein-Stra\u00dfe 23 , ,","place":["Rostock, Germany, 18059"]},{"name":"Pidstryhach Institute for Applied Problems of Mechanics and Mathematics of NAS of Ukraine , Naukowa str. 3b , ,","place":["Lviv, Ukraine, 79060"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2026,10,12]]},"reference":[{"key":"2026102808082430500_cit1","doi-asserted-by":"publisher","DOI":"10.1103\/PhysRevLett.90.077405","article-title":"Electromagnetic Wave\nPropagation in Media with Indefinite Permittivity and Permeability\nTensors","volume":"90","author":"Smith","year":"2003","journal-title":"Physical review letters"},{"key":"2026102808082430500_cit2","doi-asserted-by":"publisher","first-page":"F38","DOI":"10.1364\/JOSAB.36.000F38","article-title":"Optics\nwith hyperbolic materials","volume":"36","author":"Takayama","year":"2019","journal-title":"Journal of the Optical\nSociety of America B"},{"key":"2026102808082430500_cit3","doi-asserted-by":"publisher","first-page":"17096","DOI":"10.1039\/D2NR04181B","article-title":"Two-dimensional natural hyperbolic materials: from\npolaritons modulation to applications","volume":"14","author":"Jia","year":"2022","journal-title":"Nanoscale"},{"key":"2026102808082430500_cit4","doi-asserted-by":"publisher","first-page":"25406","DOI":"10.1364\/OE.23.025406","article-title":"Compendium\nof natural hyperbolic\nmaterials","volume":"23","author":"Korzeb","year":"2015","journal-title":"Opt. 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Rep."},{"key":"2026102808082430500_cit33","doi-asserted-by":"publisher","first-page":"17770","DOI":"10.1021\/am504296y","article-title":"Tuning the localized surface plasmon\nresonance in Cu2\u2013xSe nanocrystals by postsynthetic ligand exchange","volume":"6","author":"Balitskii","year":"2014","journal-title":"ACS Appl. Mater. Interfaces"},{"key":"2026102808082430500_cit34","doi-asserted-by":"publisher","first-page":"17630","DOI":"10.1021\/ja409754v","article-title":"Copper sulfide\nnanocrystals with\ntunable composition by reduction of covellite nanocrystals with Cu+\nions","volume":"135","author":"Xie","year":"2013","journal-title":"J. Am. Chem. Soc."},{"key":"2026102808082430500_cit35","doi-asserted-by":"publisher","first-page":"7892","DOI":"10.1038\/srep07892","article-title":"Effective medium theory\nfor anisotropic metamaterials","volume":"5","author":"Zhang","year":"2015","journal-title":"Sci. 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Phys. Lett."},{"key":"2026102808082430500_cit41","doi-asserted-by":"publisher","first-page":"24994","DOI":"10.1364\/OE.22.024994","article-title":"Surface\nscattering contribution to the plasmon width in embedded Ag nanospheres","volume":"22","author":"Carmina Monreal","year":"2014","journal-title":"Opt. 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Phys. Lett."},{"key":"2026102808082430500_cit45","doi-asserted-by":"publisher","DOI":"10.1103\/PhysRevB.55.14527","article-title":"Cu 63 NMR\nstudies of copper sulfide","volume":"55","author":"Saito","year":"1997","journal-title":"Phys. Rev. B"},{"key":"2026102808082430500_cit46","doi-asserted-by":"publisher","first-page":"20640","DOI":"10.1039\/C8NR06738D","article-title":"Copper sulfide nanosheets\nwith shape-tunable plasmonic properties in the NIR region","volume":"10","author":"Lesyuk","year":"2018","journal-title":"Nanoscale"},{"key":"2026102808082430500_cit47","first-page":"504","article-title":"The crystal\nstructure of covellite, cuse and klockmannite, cuse","volume":"39","author":"Berry","year":"1954","journal-title":"American Mineralogist: Journal of Earth and Planetary Materials"},{"key":"2026102808082430500_cit48","first-page":"996","article-title":"Crystal structure\nrefinement of covellite","volume":"61","author":"Evans","year":"1976","journal-title":"Am. Mineral."},{"key":"2026102808082430500_cit49","doi-asserted-by":"publisher","first-page":"10461","DOI":"10.1088\/0953-8984\/4\/50\/034","article-title":"Experimental\nand theoretical investigation\nof the crystal structure of CuS","volume":"4","author":"Gotsis","year":"1992","journal-title":"J. Phys.: Condens.\nMatter"},{"key":"2026102808082430500_cit50","doi-asserted-by":"publisher","first-page":"4510","DOI":"10.1103\/PhysRevLett.80.4510","article-title":"Ab initio calculation\nof excitonic effects in the optical spectra of semiconductors","volume":"80","author":"Albrecht","year":"1998","journal-title":"Physical review letters"},{"key":"2026102808082430500_cit51","doi-asserted-by":"publisher","DOI":"10.1103\/PhysRevB.57.R9385","article-title":"Theory of optical absorption in diamond,\nSi, Ge, and GaAs","volume":"57","author":"Benedict","year":"1998","journal-title":"Phys. Rev. B"},{"key":"2026102808082430500_cit52","doi-asserted-by":"publisher","first-page":"601","DOI":"10.1103\/RevModPhys.74.601","article-title":"Electronic\nexcitations: density-functional\nversus many-body Green\u2019s-function approaches","volume":"74","author":"Onida","year":"2002","journal-title":"Reviews of modern physics"},{"key":"2026102808082430500_cit53","doi-asserted-by":"publisher","first-page":"3078","DOI":"10.1039\/C7CE00203C","article-title":"Surfaces\nand morphologies of covellite (CuS) nanoparticles by means of ab initio\natomistic thermodynamics","volume":"19","author":"Morales-Garci\u0301a","year":"2017","journal-title":"CrystEngComm"},{"key":"2026102808082430500_cit54","doi-asserted-by":"publisher","first-page":"044702","DOI":"10.1063\/1.4890374","article-title":"A theoretical\ninvestigation of the\n(0001) covellite surfaces","volume":"141","author":"Gaspari","year":"2014","journal-title":"J. Chem. Phys."}],"container-title":["Nano Letters"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/pubs.acs.org\/doi\/pdf\/10.1021\/acs.nanolett.6c02585","content-type":"application\/pdf","content-version":"vor","intended-application":"unspecified"},{"URL":"https:\/\/pubs.acs.org\/nalefd\/article-pdf\/26\/42\/14551\/68814475\/acs.nanolett.6c02585.pdf","content-type":"application\/pdf","content-version":"vor","intended-application":"syndication"},{"URL":"https:\/\/pubs.acs.org\/nalefd\/article-pdf\/26\/42\/14551\/68814475\/acs.nanolett.6c02585.pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,10,28]],"date-time":"2026-10-28T12:31:07Z","timestamp":1793190667000},"score":1,"resource":{"primary":{"URL":"https:\/\/pubs.acs.org\/nalefd\/article\/26\/42\/14551\/5448687\/Natural-Hyperbolic-Dispersion-in-Anisotropic-Bulk"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,10,12]]},"references-count":54,"journal-issue":{"issue":"42","published-online":{"date-parts":[[2026,10,12]]},"published-print":{"date-parts":[[2026,10,28]]}},"URL":"https:\/\/doi.org\/10.1021\/acs.nanolett.6c02585","relation":{},"ISSN":["1530-6984","1530-6992"],"issn-type":[{"value":"1530-6984","type":"print"},{"value":"1530-6992","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,10,12]]}}}