Istraživanje stanja teških fermiona u Ce3Pd20Si6 preko nuklearne kvadrupolne rezonancije 105Pd

master's thesis

• Title (english): Study of heavy fermion state in Ce3Pd20Si6 by nuclear quadrupole resonance of 105Pd
• Author: Ivan Jakovac
• Mentor: Mihael Grbić (mentor)
• Committee member: Mihael Grbić (predsjednik povjerenstva)
• Committee member: Mario Basletić (član povjerenstva)
• Committee member: Danko Radić (član povjerenstva)
• Committee member: Miroslav Požek (član povjerenstva)
• Committee member: Ivan Kupčić (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2018-09-17, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics

Abstract

Provedena su mjerenja nuklearne kvadrupolne rezonancije (NQR) 105Pd jezgre na uzorku Ce3Pd20Si6 za dvije neekvivalentne pozicije, Pd(4a) i Pd(8c). Izmjereni NQR spektri za poziciju Pd(8c) pokazuju linearnu temperaturnu ovisnost u rasponu od 68 mK do 200 K te ne upućuju na pojavu magnetskih uređenja u okolini Pd(8c) pozicije. Temperaturna ovisnost spin-rešetka relaksacijskog vremena (T1) karakteristična je za paramagnetsko stanje (Curie-Weiss) sve do temperature < 1 K kada sustav ulazi u stanje teških fermiona. Mjerenja na temperaturama T < 800 mK pokazuju ponašanje relaksacijskog vremena u skladu s metalnim stanjem (Korringin zakon). 29Si NMR mjerenja potvrđuju paramagnetsku fazu do temperature T = 8:5 K te ukazuju na postojanje jakih fluktuacija u okolini Ce(4a) pozicije (skraćivanje T2 relaksacijskog vremena). Temperaturno ovisna promjena oblika Pd(4a) NQR spektara te značajno odstupanje od DFT proračuna ( –fDFT = 11:54 MHz, –f exp. ≈ 9:7 MHz) rezultata su jakih elektronskih korelacija u okolini Ce(4a) pozicije. Daljnja 105Pd NQR mjerenja otkriti će kako točno Ce(4a) lokalizirani momenti sudjeluju u realizaciji magnetskog uređenja u CPS sustavu.

Abstract (english)

At sufficiently low temperatures competing interactions give rise to an abundance of system states and phase transitions. If a phase transition is accessible in a zero temperature limit, either by changing pressure or magnetic field, it is identified with quantum critical point (QCP) [3]. Strong quantum fluctuations in the vicinity of QCP and suppression in Fermi temperature result in the formation of a quantum critical matter which could potentially be universally described. Heavy-fermion systems are ideal candidates for studying such phase transitions. A lattice of partially filled f electron shells, which act as localized magnetic moments, dipped into conduction electron sea gives rise to intriguing physics governed by competing Kondo and RKKY interactions. As a heavy-fermion material is brought close to QCP, divergence in its effective electron mass is accompanied with an abrupt change in Fermi surface and development of magnetic ordering. Thus far, the abrupt quantum-fluctuation-induced change in Fermi surface was considered to be the result of system dimensionality. Therefore, the discovery of this phenomenon in a cubic system makes for an exciting research subject. Ce3Pd20Si6 (CPS) is cubic structure metal, with two distinct cerium atoms in (4a) and (8c) crystallographic positions which demonstrates an enormous conduction electron effective mass at low temperature. In the absence of an external magnetic field, competing interactions induce antiferroquadrupolar (TQ = 470 mK) and antiferromagnetic (TN = 300 mK) orders. Previous research efforts [1, 2] indicate that Ce(8c) position is magnetically active. Magnetic field of 1 T suppresses AF order and reveals QCP with abrupt Fermi surface change. Study of CPS metal in zero magnetic field is conducted by means of 105Pd nuclear quadrupole resonance (NQR, as low as 50 mK) and 6 T NMR of 29Si nuclei. Exceptional agreement with DFT calculations confirms experimental data. Results suggest that magnetic ordering arises between Ce(4a) positions which also show strong electronic correlations.

• Keywords: sustav teških fermiona; jako korelirani elektronski sustavi; Kondo rešetka; kvantna kritična točka; kvantni fazni prijelaz; CPS; nuklearna kvadrupolna rezonancija; nuklearna magnetska rezonancija
• Language: croatian
• URN:NBN: urn:nbn:hr:217:831677
• Study programme: Title: Physics; specializations in: Research; Course: Research; Study programme type: university; Study level: integrated undergraduate and graduate; Academic / professional title: magistar/magistra fizike (magistar/magistra fizike)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Created on: 2018-11-20 14:32:03