Licentiate thesis defense

Probing quantum criticality in heavy fermion CeCoIn5

by Akash Khansili (Stockholm University)

Europe/Stockholm
AlbaNova A3:1003 - Styrelserum/KOMKO (AlbaNova Main Building)

AlbaNova A3:1003 - Styrelserum/KOMKO

AlbaNova Main Building

18
Description

Understanding the low-temperature properties of strongly correlated materials requires accurate measurement of the physical properties of these systems. Specific heat and nuclear spin-lattice relaxation are two such properties that allow the investigation of the electronic behaviour of the system.

In this thesis, nanocalorimetry is used to measure specific heat and also as the basis for an experimental approach, developed to disentangle the different contributions to specific heat at low temperatures. The method, that we call “Thermal Impedance Spectroscopy” (TISP) allows independent measurement of the electronic and nuclear specific heat at low temperatures based on the frequency response of the calorimeter. Additionally, it also enables simultaneous measurements of the nuclear spin-lattice relaxation time (T1). The nuclear spin lattice relaxation, as 1/T1T, and electronic specific heat, as C/T, provide information about the same quantity, electronic density of states, in the system. By comparing these properties in strongly correlated systems, we can obtain insights of electronic interactions.

Metallic indium is studied using thermal impedance spectroscopy from 0.3 K to 7 K at 35 T. The magnetic field dependence of nuclear spin-lattice relaxation rate is measured. Indium is a simple metallic system and the expected behaviour of the nuclear spin-lattice relaxation is similar to that of the electronic specific heat. The results of the measurement are matched with the expectation from a simple metallic system and the Nuclear Magnetic Resonance (NMR) measurements. This demonstrates the effectiveness of the new technique.

The heavy-fermion superconductor CeCoIn5 is studied using thermal impedance spectroscopy and ac-calorimetry. This material is located near a quantum critical point (QCP) bordering antiferromagnetism, as evidenced by doping studies. The nature of its quantum criticality and unconventional superconductivity is still elusive. Contrasting specific heat and nuclear spin-lattice relaxation in this correlated system helps to reveal the character of its quantum criticality.

The quantum criticality in CeCoIn5 is also studied using X-ray Absorption Spectroscopy (XAS) across the superconducting transition and X-ray Magnetic Circular Dichroism (XMCD) at 0.1 K and 6 T. The element-specific probe zooming in on cerium in this material indicate two things, a mixed valence of Ce in the superconducting state and a very small magnetic moment, that implies resonance-bond like antiferromagnetic local ordering in the system.

Plugin type
zoom
ID
239-996-391
Join URL