Iridates, Black Phosphorus, Weyl and Dirac Semimetals Under Extreme Conditions : Raman Study

Abstract

This thesis presents Raman studies of Sodium iridates (Na2IrO3 and Na4Ir3O8), VO2, black phosphorus, Weyl (TaAs, TaP, NbP and NbAs) and Dirac (Cd3As2) semimetals under ex-treme conditions like low temperature (upto 4K), ultra high pressures (upto 30 GPa) and electrochemical top gating of nanodevices. Temperature-dependent Raman studies were car-ried out to look for Raman signatures of Kitaev quantum spin liquid state in sodium iridates. In-situ Raman measurements were done on electrochemically top-gated field effect transistor devices fabricated on phosphorene and VO2 nanofilms to show symmetry-dependent phonon renormalization in phosphorene and microscopic origin of insulator to metal transition in VO2 nanofilms. High pressure studies were carried out to look for electronic topological as well as structural phase transitions in black phosphorus, Weyl and Dirac semimetals. For structural characterization as a function of pressure, x-ray diffraction measurements using synchrotron source have been pursued. We provide an overview of our work on these systems chapter-wise. In Chapter 1, we give a brief introduction of the systems studied in this thesis, i.e. Sodium iridates (Na2IrO3 and Na4Ir3O8), VO2, black phosphorus, Weyl (TaAs, TaP, NbP and NbAs) and Dirac (Cd3As2) semimetals. A summary of main results is given at the end of this chapter. Chapter 2 gives a brief introduction to Raman scattering and Synchrotron x-ray diffrac-tion, followed by the effect of temperature and pressure on the lattice. Further, the experi-mental setups and techniques used in this thesis are discussed. In Chapter 3, we discuss our Raman results on Sodium iridates (Na2IrO3 and Na4Ir3O8). This chapter is divided into two parts. In Part 3.1, we present temperature dependent Raman results on single crystals of (Na1−xLix)2IrO3 (x = 0, 0.05 and 0.15) in the temperature range from 4K to 300K. Our study shows a polarization independent broad band at ∼ 2750 cm−1 with a large band-width of ∼ 1800 cm−1. For Na2IrO3 the broad band is seen for temperatures ≤ 200 K and persists inside the magnetically ordered state. For Li-doped samples, the intensity of this mode increases, shifts to lower energy, and persists to higher temperatures. Such a mode has been predicted recently as a signature of the Kitaev quantum spin liquid (QSL). We assign the observed broad band to be a signature of strong Kitaev-exchange correlations. The fact that the broad band persists even inside the magnetically ordered state (with Neel temperature TN ) suggests that dynamically fluctuating moments survive even below TN . This is further supported by our mean field calculations of our collaborators. The Raman response calculated in mean field theory shows that the broad band predicted for the QSL state survives in the magnetically ordered state near the zigzag-spin liquid phase boundary. A comparison with the theoretical model gives an estimate of the Kitaev exchange interaction parameter to be JK ≈ 57 meV. In Part 3.2, we present temperature dependent Raman scattering on hyperkagome iri-date Na4Ir3O8 in the temperature range from 77K to 300K. Combining Raman scattering measurements with mean field calculations of the Raman response, we show that Kitaev-like magnetic exchange is dominant in the hyperkagome iridate Na4Ir3O8. In the measurements, we observe a broad Raman band at ∼ 3500 cm−1 with a band-width of ∼ 1700 cm−1. Calcu-lations of the Raman response of the Kitaev-Heisenberg model on the hyperkagome lattice by our collaborators show that the experimental observations are consistent with calculated Raman response where Kitaev exchange interaction (JK ) is much larger than the Heisenberg term J1 (J1/JK ∼ 0.1). A comparison with the theoretical model gives an estimate of the Kitaev exchange interaction parameter. In Chapter 4, in-situ Raman scattering studies of electrochemically top-gated VO2 ultra thin film are presented to address metal-insulator transition (MIT) under gating. The room temperature monoclinic insulating phase goes to a metallic state at a gate voltage of 2.6 V. However, the number of Raman modes do not change with electrolyte gating, showing that the metallic phase under gating is still monoclinic. The high frequency Raman mode Ag(7) −1 ˚ near 616 cm ascribed to V-O vibrations of bond length 2.06 A in VO6 octahedra hardens with increasing gate voltage and the Bg(3) mode near 654 cm−1 softens. This shows that the distortion of the VO6 octahedra in the monoclinic phase decreases with gating. The time dependent Raman data at fixed gate voltages of 1 V (for 50 minute, showing enhancement of conductivity by a factor of 50) and 2 V (for 130 minute, showing further increase in conductivity by a factor of 5) show similar changes in high frequency Raman modes Ag(7) and Bg(3) as observed in gating. This slow change in conductance together with Raman frequency changes show that the governing mechanism for metalization is more likely to be the diffusion controlled oxygen vacancy formation due to the applied electric field, rather than the carrier doping effect. In Chapter 5, we discuss our Raman studies on black phosphorus. This chapter has two parts. Part 5.1: We investigated an electrochemically top-gated field effect transistor of phos-phorene using in-situ Raman scattering. Our experiments show that the phonons with Ag symmetry depend much more strongly on the concentration of electrons than that of holes, while the phonons with Bg symmetry are insensitive to doping. With first-principles theo-retical analysis, we show that the observed electon-hole asymmetry arises from the radically different constitution of its conduction and valence bands involving π and σ bonding states respectively, whose symmetry permits coupling with only the phonons that preserve the lattice symmetry. Thus, Raman spectroscopy is shown to be a non-invasive tool for mea-suring electron concentration in phosphorene-based nanoelectronic devices, like in other 2D nanomaterials. Part 5.2: We discuss high pressure Raman experiments of Black phosphorus (BP) up to 24 GPa. The linewidths of first order Raman modes A1g, B2g and A2g of the orthorhombic phase show a minimum at 1.1 GPa. Our first-principles density functional analysis reveals that this is associated with the anomalies in electron-phonon coupling at the semiconduc-tor to topological insulator transition through inversion of valence and conduction bands, marking a change from trivial to nontrivial electronic topology. The frequencies of B2g and A2g modes become anomalous in the rhombohedral phase at 7.4 GPa, and new modes ap-pearing in the rhombohedral phase show anomalous softening with pressure. This is shown to originate from unusual structural evolution of black phosphorous with pressure, based on first-principles theoretical analysis. In Chapter 6, we describe our studies on Weyl semimetals. This chapter is divided into three parts. Part 6.1: We present high pressure Raman, synchrotron x-ray diffraction and electri-cal transport studies on Weyl semimetals NbP and TaP along with first-principles density functional theoretical (DFT) analysis. The frequencies of first-order Raman modes of NbP harden with increasing pressure and exhibit a slope change at Pc ∼ 9 GPa. The pressure dependent resistivity exhibits a minimum at Pc. The temperature coefficient of resistivity below Pc is positive as expected for semimetals but changes significantly in the high pressure phase. Using DFT calculations, we show that these anomalies are associated with pressure induced Lifshitz transition which involves appearance of electron and hole pockets in elec-tronic structure. In contrast, results of Raman and synchrotron x-ray diffraction experiments on TaP and DFT calculations show that TaP is quite robust under pressure and does not undergo any phase transition. Part 6.2: High pressure Raman, resistivity and synchrotron x-ray diffraction studies on Weyl semimetals NbAs and TaAs have been carried out along with density functional theoretical (DFT) analysis to explain pressure induced structural and electronic topologi-cal phase transitions. The frequencies of first order Raman modes harden with increasing pressure, exhibiting a slope change at PNbc∼15 GPa for NbAs and PTc a∼16 GPa for TaAs. The resistivities of NbAs and TaAs exhibit a minimum at pressures close to these transition pressures and also a change in the bulk modulus is observed. Our first-principles calculations reveal that the transition is associated with a Lifshitz transition at PNbc for NbAs while it is a structural phase transition from body centered tetragonal to hexagonal phase at PTc a for TaAs. Part 6.3: We report temperature dependent Raman study on Weyl semimetals NbAs, TaAs, TaP and NbP in the temperature range from 4K to 300K. Our study reveals phonon anomalies in the Raman frequencies of all the Raman modes in all the four systems at low temperatures. These anomalies in phonon frequencies were attributed to the addi-tional electron-phonon coupling, which gets suppressed at higher temperatures due to Pauli-blocking of interband transitions across the Weyl node. In Chapter 7, we present our Raman results on Cd3As2. This chapter has two parts. Part 7.1: We discuss high pressure Raman study of Cd3As2, a three dimensional Dirac semimetal, upto 19 GPa at room temperature. Our study shows that light scattering by in-tervalley and intravalley density fluctuations give rise to electronic Raman scattering (ERS), with Lorentzian like lineshape at low frequency. The strength and linewidth of the ERS are pressure dependent and exhibit a significant drop at Pc1 = 2.5 GPa, signifying a breakdown of the Dirac semimetal to a semiconducting phase. The first phase transition at Pc1 is also clearly identified by the significant changes in the pressure derivatives of the Raman phonon frequencies and their linewidths. Pressure dependence of phonon parameters also reveal a second phase transition at Pc2=9.5 GPa, being reported for the first time. This semicon-ductor to semiconductor transition coincides with the abrupt changes seen in the reported activation energy (band gap) of the semiconductor phase. Part 7.2: We present temperature dependent Raman study on single crystals of Cd3As2 in the temperature range from 300K to 77K. Here also, we see a broad electronic background on top of which Raman modes are superimposed. This electronic background decreases with decreasing temperature. The phonon frequencies of all the Raman modes exhibit anoma-lous behavior above 150K: T<150K, the slope of the frequency dω/dT<0, whereas above T>150K, dω/dT is close to zero or slightly positive. These phonon anomalies may be asso-ciated with the breakdown of Wiedemann-Franz law at higher temperatures. Chapter 8 summarizes our main results on different systems studied in this thesis.