NMR study of quantum criticality in Ce3Pd20Si6 AND m-PhNO2BNO systems

doctoral thesis

• Title (croatian): NMR istraživanje kvantne kritičnosti u Ce3Pd20Si6 i m-PhNO2BNO sustavima
• Author: Ivan Jakovac
• Mentor: Mihael S. Grbić (mentor)
• Committee member: Danko Radić (predsjednik povjerenstva)
• Committee member: Mihael S. Grbić (član povjerenstva)
• Committee member: Mirta Herak (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2024-10-15, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics
• Universal decimal classification (UDC): 53 - Physics

Abstract

The studied heavy fermion Ce3Pd20Si6 (CPS) and the quantum magnetic m-PhNO2BNO (BoNO) systems feature a rich phase diagram at low temperatures due to competing electronic interactions. This NMR/NQR study reveals the microscopic structure of magnetically ordered phases and characterises the quantum phase transitions (QPTs). Both systems can be driven through quantum critical points (QCPs) using an external magnetic field, with similar effects observed in other systems under hydrostatic pressure or doping. Near the QCP, strong quantum fluctuations lead to the formation of quantum critical matter, hypothesised to have a universal description. With two distinct cerium magnetic moments in CPS, antiferroquadrupolar (TQ = 470 mK) and antiferromagnetic (TN = 250 mK) orders are observed at low temperatures. Neutron scattering research links the magnetic activity to cerium moments at the Ce(8c) position, and CPS shows a Kondo-breakdown QCP, where magnetic order suppression coincides with Fermi surface reconstruction. We investigated CPS using 105Pd NQR and 29Si NMR, determined the quadrupolar coupling parameters νQ and η, and found no conventional magnetic order down to 70 mK. However, temperature-dependent relaxation rates suggest the presence of two Kondoscreening energy scales, with no sign of quantum criticality at low temperatures, implying an unobserved magnetic order between Ce(4a) moments. BoNO, an organic biradical, forms an effective S =1 Haldane state, confirmed by bulk magnetisation and X-band EPR measurements. We investigated its magnetic field-induced long-range ordered (LRO) phase using 1H NMR, determining critical exponents consistent with a 3D-XY antiferromagnetic QPT (ν = 2/3). In the gapless phase, nuclear relaxation rates align with the Tomonaga-Luttinger liquid model. The staggered magnetic moment evolution in the LRO regime matched the mean-field and Quantum Monte Carlo (QMC) predictions, though slight discrepancies suggest magnetostriction effects under an external magnetic field. Finally, we compared BoNO with other field-induced Bose-Einstein condensate systems and used the standard scaling relations to describe quasiparticle behaviour in the quantum critical region.

Abstract (croatian)

U ovom su radu istraživani spojevi iz klase sustava teških fermiona i kvantnih magneta, Ce3Pd20Si6 (CPS) i m-PhNO2BNO (BoNO). Proučavana je njihova struktura, struktura magnetski uređenih faza, spinska dinamika i priroda uočenih faznih prijelaza. Kvantni fazni prijelazi (KFP) pojavljuju se pri temperaturama od T = 0 K kada zbog suprotstavljenih elektronskih interakcija sustav mijenja svoje osnovno stanje. KFP proučavani u ovom radu mijenjaju osnovno stanje sustava iz magnetski uređenoga (antiferomagnetskog) u neuređeno paramagnetsko i/ili metalno stanje (Fermijeva tekućina). Tehnikom nuklearne magnetske i kvadrupolne rezonancije okarakterizirana su magnetski uređena stanja u ovim sustava, te kvantni fazni prijelaz kada primjenom vanjskog magnetskog polja sustave tjeramo kroz kvantnu kritičnu točku (KKT). U sličnim sustavima kvantni fazni prijelazi su također inducirani primjenom hidrostatičkog tlaka ili kemijskog dopiranja. Univerzalni teorijski opis kvantno kritičnog ponašanja u blizini KKT neovisan je o eksperimentalnoj realizaciji (tj. uzorku) kvantnog magnetskog sustava. Poznato je da takav univerzalni opis ovisi isključivo o dimenzionalnosti sustava i prirodi osnovnih stanja (magnetsko uređenje, orbitalno uređenje) s obje strane KKT.

• Keywords: quantum phase transition; quantum criticality; nuclear magnetic resonance; nuclear quadrupolar resonance; heavy fermions; two-component Kondo screening; CEF splitting; low-dimensional systems; quantum magnetism; Haldane system; Tomonaga-Luttinger model; XY-AFM; Bose-Einstein condensate
• Keywords (croatian): kvantni fazni prijelaz; kvantna kritičnost; nuklearna kvadrupolna rezonancija; nuklearna magnetska rezonancija; sustav teških fermiona; Kondovo zasjenjenje; cijepanje kristalnim električnim poljem; niskodimenzionalni sustavi; kvantni magnetizam; Haldaneov sustav; Tomonaga-Luttinger model; Bose-Einsteinov kondenzat
• Language: english
• URN:NBN: urn:nbn:hr:217:676520
• Promotion: 2024
• Project: Number: IP-2018-01-2970; Title: Mikroskopska istraživanja induciranih faza u jako koreliranim elektronskim sustavima; Title: Microscopic study of induced phases in Strongly correlated electronic systems; Acronym: MicroS; Leader: Miroslav Požek; Jurisdiction: Croatia; Funder: Hrvatska zaklada za znanost; Funding stream: Research Projects
• Study programme: Title: Doctoral study; Study programme type: university; Study level: postgraduate; Academic / professional title: doktor/doktorica znanosti u području prirodnih znanosti (doktor/doktorica znanosti u području prirodnih znanosti)
• Type of resource: Text
• Extent: 145 str.
• File origin: Born digital
• Access conditions: Embargoed access; Embargo expiration date: 2025-10-17
• Created on: 2024-11-19 11:33:52