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Author: Caglieris, Federico ×

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    Calorimetric evidence for two phase transitions in Ba1−xKxFe2As2 with fermion pairing and quadrupling states
    ([London] : Nature Publishing Group UK, 2023) Shipulin, Ilya; Stegani, Nadia; Maccari, Ilaria; Kihou, Kunihiro; Lee, Chul-Ho; Hu, Quanxin; Zheng, Yu; Yang, Fazhi; Li, Yongwei; Yim, Chi-Ming; Hühne, Ruben; Klauss, Hans-Henning; Putti, Marina; Caglieris, Federico; Babaev, Egor; Grinenko, Vadim
    Materials that break multiple symmetries allow the formation of four-fermion condensates above the superconducting critical temperature (T c). Such states can be stabilized by phase fluctuations. Recently, a fermionic quadrupling condensate that breaks the Z 2 time-reversal symmetry was reported in Ba1−xKxFe2As2. A phase transition to the new state of matter should be accompanied by a specific heat anomaly at the critical temperature where Z 2 time-reversal symmetry is broken (TcZ2>Tc). Here, we report on detecting two anomalies in the specific heat of Ba1−xKxFe2As2 at zero magnetic field. The anomaly at the higher temperature is accompanied by the appearance of a spontaneous Nernst effect, indicating the breakdown of Z 2 symmetry. The second anomaly at the lower temperature coincides with the transition to a zero-resistance state, indicating the onset of superconductivity. Our data provide the first example of the appearance of a specific heat anomaly above the superconducting phase transition associated with the broken time-reversal symmetry due to the formation of the novel fermion order.
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    Elastoresistivity of Heavily Hole-Doped 122 Iron Pnictide Superconductors
    (Lausanne : Frontiers Media, 2022) Hong, Xiaochen; Sykora, Steffen; Caglieris, Federico; Behnami, Mahdi; Morozov, Igor; Aswartham, Saicharan; Grinenko, Vadim; Kihou, Kunihiro; Lee, Chul-Ho; Büchner, Bernd; Hess, Christian
    Nematicity in heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from magnetic instability were declared in RbFe2As2 and its sister compounds. Here, we report a systematic elastoresistance study of a series of isovalent- and electron-doped KFe2As2 crystals. We found divergent elastoresistance on cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven toward a Lifshitz transition. However, we conclude that none of them necessarily indicates an independent nematic critical point. Instead, the increased nematicity can be associated with another electronic criticality. In particular, we propose a mechanism for how elastoresistivity is enhanced at a Lifshitz transition.
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    Highly efficient modulation doping: A path toward superior organic thermoelectric devices
    (Washington, DC [u.a.] : Assoc., 2022) Wang, Shu-Jen; Panhans, Michel; Lashkov, Ilia; Kleemann, Hans; Caglieris, Federico; Becker-Koch, David; Vahland, Jörn; Guo, Erjuan; Huang, Shiyu; Krupskaya, Yulia; Vaynzof, Yana; Büchner, Bernd; Ortmann, Frank; Leo, Karl
    We investigate the charge and thermoelectric transport in modulation-doped large-area rubrene thin-film crystals with different crystal phases. We show that modulation doping allows achieving superior doping efficiencies even for high doping densities, when conventional bulk doping runs into the reserve regime. Modulation-doped orthorhombic rubrene achieves much improved thermoelectric power factors, exceeding 20 μW m−1 K−2 at 80°C. Theoretical studies give insight into the energy landscape of the heterostructures and its influence on qualitative trends of the Seebeck coefficient. Our results show that modulation doping together with high-mobility crystalline organic semiconductor films is a previosly unexplored strategy for achieving high-performance organic thermoelectrics.