start-ver=1.4 cd-journal=joma no-vol=109 cd-vols= no-issue=20 article-no= start-page=L201103 end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2024 dt-pub=20240503 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Realization of nodal-ring semimetal in pressurized black phosphorus en-subtitle= kn-subtitle= en-abstract= kn-abstract=Topological semimetals are intriguing targets for exploring unconventional physical properties of massless fermions. Among them, nodal-line or nodal-ring semimetals have attracted attention for their unique one-dimensional band contact in momentum space and resulting nontrivial quantum phenomena. By field angular resolved magnetotransport measurements and theoretical calculations, we show that pressurized black phosphorus (BP) is an ideal nodal-ring semimetal with weak spin-orbit coupling, which has a sole and carrier density-tunable nodal ring isolated from other trivial bands. We also revealed that the large magnetoresistance effect and its field-angular dependence in semimetallic BP are due to highly anisotropic relaxation time. Our results establish pressurized BP as an elemental model material for exploring nontrivial quantum properties unique to the topological nodal ring. en-copyright= kn-copyright= en-aut-name=AkibaKazuto en-aut-sei=Akiba en-aut-mei=Kazuto kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=AkahamaYuichi en-aut-sei=Akahama en-aut-mei=Yuichi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=TokunagaMasashi en-aut-sei=Tokunaga en-aut-mei=Masashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= en-aut-name=KobayashiTatsuo C. en-aut-sei=Kobayashi en-aut-mei=Tatsuo C. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=4 ORCID= affil-num=1 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= affil-num=2 en-affil=Graduate School of Science, University of Hyogo kn-affil= affil-num=3 en-affil=The Institute for Solid State Physics, The University of Tokyo kn-affil= affil-num=4 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= END start-ver=1.4 cd-journal=joma no-vol=107 cd-vols= no-issue=24 article-no= start-page=245117 end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2023 dt-pub=20230609 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Phonon-mediated superconductivity in the Sb square-net compound LaCuSb2 en-subtitle= kn-subtitle= en-abstract= kn-abstract=We investigated the electronic structure and superconducting properties of single-crystalline LaCuSb2. The resistivity, magnetization, and specific heat measurements showed that LaCuSb2 is a bulk superconductor. The observed Shubnikov?de Haas oscillation and magnetic field dependence of the Hall resistivity can be reasonably understood assuming a slightly hole-doped Fermi surface. An electron-phonon coupling calculation clarified the difference from the isostructural compound LaAgSb2, indicating that (i) low-frequency vibration modes related to the interstitial layer sandwiched between the Sb square nets significantly contribute to the superconductivity and (ii) carriers with sizable electron-phonon coupling distribute isotropically on the Fermi surface. These are assumed to be the origin of the higher superconducting transition temperature compared with LaAgSb2. We conclude that the superconducting properties of LaCuSb2 can be understood within the framework of the conventional phonon-mediated mechanism. en-copyright= kn-copyright= en-aut-name=AkibaKazuto en-aut-sei=Akiba en-aut-mei=Kazuto kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=KobayashiTatsuo C. en-aut-sei=Kobayashi en-aut-mei=Tatsuo C. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= affil-num=1 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= affil-num=2 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= END start-ver=1.4 cd-journal=joma no-vol=106 cd-vols= no-issue=16 article-no= start-page=L161113 end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2022 dt-pub=20221027 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Observation of superconductivity and its enhancement at the charge density wave critical point in LaAgSb 2 en-subtitle= kn-subtitle= en-abstract= kn-abstract=We discover superconductivity (SC) in LaAgSb2 at ambient pressure and its close correlation with a charge density wave (CDW) under pressure. The superconducting transition temperature (Tc) exhibits a sharp peak at a CDW critical pressure of 3.2 GPa. We demonstrate that the carriers inhabiting the Sb-square net are crucial not only in the formation of CDW but also in SC for their relatively strong electron-phonon coupling (EPC). Furthermore, theoretical EPC strength in pristine LaAgSb 2 cannot explain the observed peak with Tc?1 K, which indicates that an additional mechanism reinforces SC only around the CDW critical pressure. en-copyright= kn-copyright= en-aut-name=AkibaKazuto en-aut-sei=Akiba en-aut-mei=Kazuto kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=UmeshitaNobuaki en-aut-sei=Umeshita en-aut-mei=Nobuaki kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=KobayashiTatsuo C. en-aut-sei=Kobayashi en-aut-mei=Tatsuo C. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= affil-num=1 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= affil-num=2 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= affil-num=3 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= END start-ver=1.4 cd-journal=joma no-vol=105 cd-vols= no-issue=3 article-no= start-page=035108 end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2022 dt-pub=202216 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Magnetotransport studies of the Sb square-net compound LaAgSb2 under high pressure and rotating magnetic fields en-subtitle= kn-subtitle= en-abstract= kn-abstract=Square-net-layered materials have attracted attention as an extended research platform of Dirac fermions and of exotic magnetotransport phenomena. In this study, we investigated the magnetotransport properties of LaAgSb2, which has Sb-square-net layers and shows charge density wave (CDW) transitions at ambient pressure. The application of pressure suppresses the CDWs, and above a pressure of 3.2 GPa a normal metallic phase with no CDWs is realized. By utilizing a mechanical rotator combined with a high-pressure cell, we observed the angular dependence of the Shubnikov?de Haas (SdH) oscillation up to 3.5 GPa, and we confirmed the notable two-dimensional nature of the Fermi surface. In the normal metallic phase, we also observed a remarkable field-angular-dependent magnetoresistance (MR), which exhibited a gbutterflylikeh polar pattern. To understand these results, we theoretically calculated the Fermi surface and conductivity tensor at the normal metallic phase. We showed that the SdH frequency and Hall coefficient calculated based on the present Fermi surface model agree well with the experiment. The transport properties in the normal metallic phase are mostly dominated by the anisotropic Dirac band, which has the highest conductivity due to linear energy dispersions.We also proposed that momentum-dependent relaxation time plays an important role in the large transverse MR and negative longitudinal MR in the normal metallic phase, which is experimentally supported by the considerable violation of Kohlerfs scaling rule. Although quantitatively complete reproduction was not achieved, the calculation showed that the elemental features of the butterfly MR could be reasonably explained as the geometrical effect of the Fermi surface. en-copyright= kn-copyright= en-aut-name=AkibaKazuto en-aut-sei=Akiba en-aut-mei=Kazuto kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=UmeshitaNobuaki en-aut-sei=Umeshita en-aut-mei=Nobuaki kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=KobayashiTatsuo C. en-aut-sei=Kobayashi en-aut-mei=Tatsuo C. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= affil-num=1 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= affil-num=2 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= affil-num=3 en-affil=Graduate School of Natural Science and Technology, Okayama University kn-affil= END start-ver=1.4 cd-journal=joma no-vol=103 cd-vols= no-issue= article-no= start-page=085134 end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2021 dt-pub=20210222 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Successive destruction of charge density wave states by pressure in LaAgSb2 en-subtitle= kn-subtitle= en-abstract= kn-abstract=We comprehensively studied the magnetotransport properties of LaAgSb2 under high pressure up to 4 GPa, which showed unique successive charge density wave (CDW) transitions at TCDW1?210 K and TCDW2?190 K at ambient pressure. With the application of pressure, both TCDW1 and TCDW2 were suppressed and disappeared at the critical pressures of PCDW1=3.0?3.4 GPa and PCDW2=1.5?1.9 GPa, respectively. At PCDW1, the Hall conductivity showed a steplike increase, which is consistently understood by the emergence of a two-dimensional hollow Fermi surface at PCDW1. We also observed a significant negative magnetoresistance effect when the magnetic field and current were applied parallel to the c axis. The negative contribution was observed in the whole pressure region from 0 to 4 GPa. Shubnikov?de Haas (SdH) oscillation measurements under pressure directly showed the changes in the Fermi surface across the CDW phase boundaries. In PPCDW1, we observed a single frequency of ?48 T with a cyclotron effective mass of 0.066m0, whose cross section in the reciprocal space corresponded to only 0.22% of the first Brillouin zone. Besides, we observed another oscillation component with frequency of ?9.2 T, which is significantly enhanced in the limited pressure range of PCDW2