Ultrafast dynamics of strongly excited corelated states

Naseska, Mimoza (author), Mertelj, Tomaž (mentor)

Electronic correlations alone or in combination with lattice degrees of freedom can often lead to existence of variety of exotic ground states like high temperature superconductivity, charge density wave ordering, colossal magnetoresistance etc. Different phases of a correlated system can often be transformed into each other by subtle external perturbations. An example of such perturbation is a laser pulse with duration of several tens of fs. Short pulses of this kind can initiate a phase transition from a ground state of the material to some final state which in some cases can be metastable. Using ultrafast time resolved spectroscopy we explored the dynamics of the first order phase transition between the insulating and metallic state of CuIr2S4. We showed that the structural dynamics in CuIr2S4 is dominated by the first-order transition nucleation kinetics which prevents the complete transition to the high temperature phase at large excitation densities on ultrafast (~10 ps) timescales. Additionally, we investigated the possibility of a photoinducing phase transition to a hidden or a metastable state. However, we have found no evidence that a femtosecond excitation could lead to a long-lived metastable state that is more conducting than the already known slowly-formed disordered weakly-conducting state. The existence of large spin-orbit coupling in materials allows us to study the photoinduced magnetic dynamics using short laser pulses. CeSb is a rare-earth pnictogen that has large spin-orbit coupling and quite complicated magnetic phase diagram with up to 25 different magnetic states. By exploiting the magneto-optical phenomena we studied the photoinduced excitations in CeSb on ultrafast timescales in the different phases that arise by changing the temperature or the external magnetic field. The qualitative and quantitative differences of the measured responses on ultrafast timescales in the different magnetic phases allowed us to construct a phase diagram of photoinduced magnetic excitations as a function of temperature and magnetic field.

• Language: English
• Keywords: ultrafast spectroscopy, phase transitions, orbital ordering, magnetooptics, magnetic anisotropy, Raman spectroscopy, ferromagnetism, magnon scattering
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2022
• PID: 20.500.12556/RUL-142975
• COBISS.SI-ID: 132320771
• Publication date in RUL: 07.12.2022

Secondary language

• Language: Slovenian
• Title: Ultrahitra dinamika močno vzbujenih koreliranih stanj

Abstract

Elektronske korelacije v kombinaciji z mocno elektronsko-fononsko sklopitvijo lahko pripeljejo do razlicnih eksoticnih stanj snovi kot so visokotemperaturna superprevodnost, valovi gostote naboja, kolosalna magnetna upornost ipd. Zunanje perturbacije pa lahko povzrocijo prehod med razlicnimi fazami koreliranega sistema. Laserski sunek z dolžino nekaj deset femtosekund (fs) lahko sproži fazni prehod iz osnovnega v neko koncno stanje koreliranega sistema, ki je v dolocenih primerih lahko metastabilno. V tej disertaciji sta predstavljeni raziskavi dinamike fotovzbujenih faznih prehodov v materialih CuIr2S4 in CeSb na femtosekundni casovni skali z uporabo ultrahitre casovno locljive opticne spektroskopije in magnetometrije. Raziskali smo dinamiko fotoinduciranega faznega prehoda prvega reda med kovinskim in izolatorskim stanjem v CuIr2S4 na ultrahitri casovni skali. Pokazali smo, da strukturna dinamika CuIr2S4, ki jo lahko opišemo z nukleacijskim kineticnim modelom, na kratkih casovnih skalah reda velikosti 10 ps preprecuje prehod v visokotempraturno fazo CuIr2S4 pri velikih gostotah fotovzbujanja. Poleg tega smo raziskali tudi obstoj fotoinduciranih skritih ali metastabilnih stanj, vendar pa nismo našli dokazov, da bi vzbujanje CuIr2S4 s kratkimi sunki vodilo do kakega dolgoživega metastabilnega stanja, ki bi imelo vecjo prevodnost kot že znano metastabilno stanje. CeSb (cerijev antimonid) je material, ki ima precej bogat magnetni fazni diagram s približno 25 razlicnimi magnetnimi stanji, ki se pojavljajo s spreminjanjem temperature ali gostote zunajnega magnetnega polja. Obstoj velike sklopitve med spinsko in tirno vrtilno kolicino v Ce smo izkoristili za preucevanje fotoinducirane dinamike razlicnih magnetnih stanj preko magneto-opticnih pojavov. Opazili smo kvalitativne in kvantitativne razlike med odzivi na fs casovni skali pri razlicnih vrednostih temperature in magnetnega polja ter iz tega reproducirali fazni diagram.

• Keywords: ultrahitra spektroskopija, fazni prehodi, ureditev orbital, magnetoopticni pojavi, magnetna anizotropija, Ramanova spektroskopija, feromagnetizem, sipanje na magnonih