O B A V I J E S T
IVAN JAKOVAC
održat će znanstveni kolokvij (doktorski seminar) iz Fizike kondenzirane tvari pod naslovom
NMR study of quantum criticality in Ce3Pd20Si6 and m-PhNO2BNO systems
(NMR istraživanje kvantne kritičnosti u Ce3Pd20Si6 i m-PhNO2BNO sustavima)
In this study, we have investigated two quantum magnetic systems, Ce3Pd20Si6 (CPS) and m-PhNO2BNO (BoNO). The research was focused on uncovering the structure of magnetically ordered phases as well as characterizing the curious spin dynamics of both ordered and disordered states using local probe techniques such as nuclear magnetic and quadrupolar resonance (NMR, NQR) and electron paramagnetic resonance (EPR). Using an external magnetic field as a driving parameter, we have induced a quantum phase transition (QPT) in BoNO, allowing us to probe a quantum critical matter modelled by quasiparticle excitations with universal physical properties.
The cubic heavy-fermion system CPS, with its two distinct Ce 4f magnetic moments, presents a system where different Kondo screening energy scales lead to interesting magnetic properties. In a zero magnetic field, the system orders antiferroquadrupolarly (TQ = 470 mK) and antiferromagnetically (TN = 250 mK) as determined from neutron scattering and magnetization measurements. We employ 105Pd NQR (down to 50 mK) and 29Si NMR to characterize the magnetically ordered phases. We have successfully determined the values of NQR frequencies for Pd(48h) and Pd(32f) sites, which were further corroborated by density functional theory (DFT) calculations. Our results show no conventional magnetic order down to T=70 mK at Ce(8c) sites. However, the temperature dependence of nuclear relaxation times T1-1 shows a crossover between two metallic regimes at T=1.5 K, where we also observe a curious two-component relaxation. The 29Si NMR measurements feature strong fluctuations at T = 5 K. Finally, the magnetic field dependence of T1-1 at the Pd(32f) site (T=128 mK) shows no sign of quantum criticality. These results suggest magnetic order between Ce(4a) 4f moments.
BoNO is a quasi-1D organic biradical whose ferromagnetically coupled unpaired electrons form an effective S=1 state. BoNO crystallizes in an orthorhombic (Pbca) unit cell where S=1 chains propagate along the crystallographic a-axis. The bulk magnetization and susceptibility measurements were used to determine BoNO's effective paramagnetic moment, dominant spin exchange coupling, and critical magnetic fields. The X-band EPR data confirms that BoNO is a Haldane system with a fully isotropic g-factor. The temperature dependence of both g-factor and EPR linewidths are consistent with models of low-dimensional magnetic systems.
Furthermore, we have successfully assigned the 1H NMR spectra and used the high-resolution NMR spectroscopy at magnetic fields up to H = 35 T to probe the system’s complete phase diagram. We observe the Tomonaga-Luttinger liquid (TLL) behaviour for the first time in the isotropic Haldane system. We have successfully implemented the Bayesian fitting methods to extract TLL parameters in a broad range of magnetic fields. We have also determined the phase boundary and the temperature evolution of the staggered magnetic moment in the long-range ordered phase. The critical exponent at QPT near Hc2 = 33.6 T, ν = 0.66, is consistent with quantum Monte Carlo calculations for a 3D-XY antiferromagnetic quantum phase transition. Finally, the standard scaling relation (H-Hc2)/T, which describes the quasiparticle behaviour in the quantum critical region, is used to compare BoNO with other known realizations of low dimensional systems. These findings on BoNO significantly contribute to our understanding of Haldane systems.
Seminar će se održati u ponedjeljak 20. svibnja 2024. godine u 11:00 sati u predavaonici F-12 Fizičkog odsjeka Prirodoslovno-matematičkog fakulteta Sveučilišta u Zagrebu.
Pozivaju se studenti doktorskog studija Fizike, smjer Fizika kondenzirane tvari, da prisustvuju seminaru ako nisu spriječeni.
Voditelj smjera
Prof. dr. sc. Denis Sunko, v. r.
Pristupačnost
