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Université de Fribourg

Muon spin rotation and infrared spectroscopy study of Ba$_{1-x}$Na$_{x}$Fe$_{2}$As$_{2}$

Sheveleva, Evgeniia ; Xu, Bing ; Marsik, Premysl ; Lyzwa, Fryderyk ; Mallett, Benjamin P. P. ; Willa, K. ; Meingast, C. ; Wolf, Th. ; Shevtsova, T. ; Pashkevich, Yu. G. ; Bernhard, Christian

In: Physical Review B, 2020, vol. 101, no. 22, p. 224515

The magnetic and superconducting properties of a series of underdoped Ba 1 − x Na x Fe 2 As 2 (BNFA) single crystals with 0.19 ≤ x ≤ 0.34 have been investigated with the complementary muon-spin-rotation ( μ SR ) and infrared spectroscopy techniques. The focus has been on the different antiferromagnetic states in the underdoped regime and their competition with superconductivity,...

Université de Fribourg

Optical signatures of multifold fermions in the chiral topological semimetal CoSi

Xu, Bing ; Fang, Zhenyao ; Sánchez-Martínez, Miguel-Angel ; Venderbos, Jorn W. F. ; Ni, Zhuoliang ; Qiu, Tian ; Manna, Kaustuv ; Wang, Kefeng ; Paglione, Johnpierre ; Bernhard, Christian ; Felser, Claudia ; Mele, Eugene J. ; Grushin, Adolfo G. ; Rappe, Andrew M. ; Wu, Liang

In: Proceedings of the National Academy of Sciences, 2020, vol. 117, no. 44, p. 27104–27110

We report the optical conductivity in high-quality crystals of the chiral topological semimetal CoSi, which hosts exotic quasiparticles known as multifold fermions. We find that the optical response is separated into several distinct regions as a function of frequency, each dominated by different types of quasiparticles. The low-frequency intraband response is captured by a narrow Drude peak...

Université de Fribourg

${\eta}$-paired superconducting hidden phase in photodoped Mott insulators

Li, Jiajun ; Golez, Denis ; Werner, Philipp ; Eckstein, Martin

In: Physical Review B, 2020, vol. 102, no. 16, p. 165136

We show that a metastable η-pairing superconducting phase can be induced by photodoping doublons and holes into a strongly repulsive fermionic Hubbard model. The doublon-hole condensate originates from an intrinsic doublon-hole exchange interaction and does not rely on the symmetry of the half-filled Hubbard model. It extends over a wide range of doublon densities and effective temperatures....

Université de Fribourg

Entropy and electronic orders of the three-orbital Hubbard model with antiferromagnetic Hund coupling

Yue, Changming ; Hoshino, Shintaro ; Werner, Philipp

In: Physical Review B, 2020, vol. 102, no. 19, p. 195103

An antiferromagnetic Hund coupling in multiorbital Hubbard systems induces orbital freezing and an associated superconducting instability, as well as unique composite orders in the case of an odd number of orbitals. While the rich phase diagram of the half-filled three-orbital model has recently been explored in detail, the properties of the doped system remain to be clarified. Here, we...

Université de Fribourg

Superconducting and antiferromagnetic properties of dual-phase V3Ga

Jamer, Michelle E. ; Wilfong, Brandon ; Buchelnikov, Vasiliy D. ; Sokolovskiy, Vladimir V. ; Miroshkina, Olga N. ; Zagrebin, Mikhail A. ; Baigutlin, Danil R. ; Naphy, Jared ; Assaf, Badih A. ; Lewis, Laura H. ; Pulkkinen, Aki ; Barbiellini, Bernardo ; Bansil, Arun ; Heiman, Don

In: Applied Physics Letters, 2020, vol. 117, no. 6, p. 062401

The binary compound V3Ga can exhibit two near-equilibrium phases, the A15 structure that is superconducting and the Heusler D03 structure that is semiconducting and antiferromagnetic. Density functional theory calculations show that these two phases are nearly degenerate, being separated in energy by only ±10 meV/atom. Our magnetization measurements on bulk-grown samples show ...

Université de Fribourg

Superconducting optical response of photodoped Mott insulators

Li, J. ; Golez, D. ; Werner, Philipp ; Eckstein, M.

In: Modern Physics Letters B, 2020, vol. 34, no. 19n20, p. 2040054

Ultrafast laser pulses can redistribute charges in Mott insulators on extremely short time scales, leading to the fast generation of photocarriers. It has recently been demonstrated that these photocarriers can form a novel η-paired condensate at low temperatures, featuring a staggered superconducting pairing field. In this conference paper, we discuss the origin of the η-paired hidden...

Université de Fribourg

Ab initio description of the ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+{\delta}}$ electronic structure

Nokelainen, Johannes ; Lane, Christopher ; Markiewicz, Robert S. ; Barbiellini, Bernardo ; Pulkkinen, Aki ; Singh, Bahadur ; Sun, Jianwei ; Pussi, Katariina ; Bansil, Arun

In: Physical Review B, 2020, vol. 101, no. 21, p. 214523

Bi-based cuprate superconductors are important materials for both fundamental research and applications. As in other cuprates, the superconducting phase in the Bi compounds lies close to an antiferromagnetic phase. Our density functional theory calculations based on the strongly-constrained-and- appropriately-normed exchange correlation functional in ...

Université de Fribourg

Superconductivity-induced transverse plasma mode and phonon anomaly in the $c$-axis response of the bilayer compound ${\mathrm{RbCa}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$

Xu, Bing ; Munzar, Dominik ; Dubroka, Adam ; Sheveleva, Evgeniia ; Lyzwa, Fryderyk ; Maršík, Premysl ; Wang, C. N. ; Wang, Z. C. ; Cao, G. H. ; Bernhard, Christian

In: Physical Review B, 2020, vol. 101, no. 21, p. 214512

We studied the infrared response of polycrystalline samples of the iron arsenide superconductor (Rb,Cs)Ca2Fe4As4F2 (Rb,Cs-12442), which has a bilayer structure similar to the high-Tc cuprates YBa2Cu3O7 (YBCO) and Bi2Sr2CaCu2O8. The c-axis reflectivity spectra Rc have been derived from the reflectivity spectra of the polycrystalline samples Rpoly and the in-plane spectrum of a corresponding...

Université de Fribourg

Mahan excitons in room-temperature methylammonium lead bromide perovskites

Palmieri, Tania ; Baldini, Edoardo ; Steinhoff, Alexander ; Akrap, Ana ; Kollár, Márton ; Horváth, Endre ; Forró, László ; Jahnke, Frank ; Chergui, Majed

In: Nature Communications, 2020, vol. 11, no. 1, p. 850

In a seminal paper, Mahan predicted that excitonic bound states can still exist in a semiconductor at electron-hole densities above the insulator-to-metal Mott transition. However, no clear evidence for this exotic quasiparticle, dubbed Mahan exciton, exists to date at room temperature. In this work, we combine ultrafast broadband optical spectroscopy and advanced many-body calculations to...

Université de Fribourg

Magneto-transport in La2/3Sr1/3MnO3/YBa2Cu3O7/Alq3/Co spin-valves

Cerreta, Andrea ; Gaina, Roxana ; Nuccio, Laura ; Marozau, Ivan ; Sen, Kaushik ; De Andrés Prada, Roberto ; Sarkar, Subhrangsu ; Bernhard, Christian

In: EPL (Europhysics Letters), 2020, vol. 129, no. 3, p. 37002

We studied how an additional layer of the high superconductor YBCO affects the magneto-transport across a LSMO/Alq3/Co spin-valve structure. We found that up to a thickness of at least 10 nm the YBCO layer on top of LSMO hardly changes the spin-valve effect, since the device resistance still depends on the relative orientation of the LSMO and Co magnetization. The spin-valve effect persists...