講師:Dr. Marcin Konczykowski (Ecole Polytechnique) JSPS外国人招へい研究者
日時:2019年11月20日(水) 10:30~12:00
場所:東京大学 柏キャンパス 基盤棟物質系講義室(2B6号室)

タイトル:Charge and Magnetic Orders in Superconductors, Competing or Intertwinned?


Superconductivity of iron-based pnictides  (IBS) arises from the background of magnetic order (Spin Density Wave, SDW) while in transition metal dichalcogenides, charge order  (Charge Density Wave, CDW) is a ground state. From one side long range of both magnetic and charge orders inhibit superconducting state, but their fluctuations contribute to formation of the Cooper pairs.

Representative example is iron-based superconducting pnictides (IBS) where substitution induced depression of antiferromagnetism (AF) of parent compound lets emerge superconducting order. Similarly, in canonical transition metal dichalcogenide NbSe2, superconductivity stem from charge density wave (CDW) state. The key issue is in what extend the charge/spin order exclude superconductivity or contribute to pairing.

In order to elucidate this question we investigated effect of disorder induced in controlled way by energetic particle irradiation on stability of ordered phases. In both types of systems, depression of charge or spin order by disorder leads to enhancement of superconducting state. In my presentation I will discuss evolution of composition – temperature phase diagram with point-like disorder induced by low temperature irradiation of 122 family of IBS and in NbSe2 [1]. In the later compound strong depression of CDW by disorder is associated with changes of phonon spectra monitored by Raman spectroscopy, including spectacular depression of Higgs mode associated peak.  Unlike in underdoped IBS, where temperature of SDW transition drops after introduction of point disorder but remain of first order, depression of CDW transition by irradiation-induced disorder in NbSe2 seems to have different nature. Even if signature of CDW transition, TCDW by kink in resistivity vs. temperature curves shows smooth decrease of TCDW, other observations: X ray diffraction and evolution of Hall coefficient points to persistence of short range CDW phase even in strongly disordered NbSe2 samples.

Recently, we observed similar behavior in TaS2, irradiation induced strong increase of superconducting transition temperature Tc from below 1K to 4K  associated with slight decrease of CDW transition inferred from anomaly in resistivity vs. temperature curves.

[1] Kuyil Cho, et al. Nature Comm. 9, 2796, (2018)