Collective Excitations in the Antisymmetric Channel of Raman Spectroscopy

Polarization resolved Raman spectroscopy is a powerful tool as it simultaneously offers exceptional symmetry and energetic resolution (∼0.1 meV), making it particularly suitable for studying low energy collective modes in systems with nontrivial electron correlation. In the condensed matter physics, Raman spectroscopy has been crucial in understanding superconductivity by studying symmetry of the Cooper pair breaking peak, electron–phonon interaction in semiconductors by studying the plasmon–polaritons, and charge density waves by studying the amplitude modes.

Since the days when C.V. Raman and coworkers discovered inelastic light scattering using Calcutta sunlight as the source in the 1920s (Raman, Indian J Phys 2:387–398, 1928), many improvements have been done on Raman spectroscopy apparatus in the past century. Raman scattering setup has evolved into very diverse forms, from portable or even handheld Raman spectrometers widely used by forensic scientists and minefield workers to gigantic state-of-the-art triple-stage grating spectrometers used in research labs. The data collection efficiency has also improved many orders of magnitude, alongside with better spectral resolution and polarization optics. This is due to improvements on almost every single element: laser light source, polarization optics, aberration corrected lens, holographic blazed gratings, off-axis parabolic mirrors, high precision slits, back illuminated CCD detectors, and many other factors (Hayes and Loudon, Scattering of light by crystals. Courier Corporation, Chelmsford, 2012; Palmer, Loewen, Diffraction grating handbook. Newport Corporation, New York, 2005). Given the diversity and breadth of this topic, it is impossible to have a complete account of all involved instrumentations. In this chapter, I will introduce the basic elements common to most low temperature polarization resolved Raman spectroscopic research labs, but focus will be given to the setups we employ here at Rutgers. Then I will discuss the data acquisition, error sources, and data analysis techniques we used in this monograph.

In this chapter, I will introduce our Raman scattering study of the “hidden order” (HO) and antiferromagnetic (AFM) phases in URu₂Si₂, a heavy fermion material of undiminished interest in the condensed matter community for over thirty years. I will first give a short overview on the heavy fermion physics and the exotic phases in this compound, where focus will be given on the experimental results [Sect. 3.1]. Then I will describe typical sample preparation and characterization procedures, which are particularly important for detecting the electronic phases in URu₂Si₂ [Sect. 3.2]. In Sect. 3.3, I will present our Raman spectroscopic results in the pristine and Fe substituted URu_2−xFe_xSi₂ samples, which are fully consistent with a minimal model that explains the origin of the unusual A_2g susceptibility in URu₂Si₂ [Sect. 3.4]. We will also discuss the origin of the collective mode in the ordered phase at low temperature and the interrelation between the HO and AFM phases.

In this chapter I will present our study of the secondary emissions (radiation) in a prototypical 3D topological insulator (TI) material, Bi₂Se₃. I will begin with a short overview on the physics of 3D TIs, with focus on the properties of the spin polarized surface states (Sect. 4.1.2). In Sect. 4.2, I will present our Raman spectroscopic results on the bulk and surface phonon modes in Bi₂Se₃, for which we identified the symmetries and self-energies and discussed the excitation dependence of them. In Sect. 4.3, I will present our electronic Raman scattering results in the much higher energy regime, where we observed a chiral spin mode as collective spin–flip excitations of the surface Dirac fermions. In Sect. 4.4, I will present our photoluminescence study of Bi₂Se₃, where we show a highly circular polarized surface exciton as a result of chiral spin texture in the surface states. Finally, I will make a brief summary of our study and its implications in Sect. 4.5.

In this monograph, we used Raman scattering to identify collective excitations in the “chiral” symmetry channel in two completely different nonmagnetic systems: the HO phase of heavy fermion metal URu₂Si₂ and the surface states of 3D topological insulator Bi₂Se₃.