Terahertz Spectroscopy for Quantum Condensed Matter Dhanvir Singh Rana

Transcription

1 Terahertz Spectroscopy for Quantum Condensed Matter Dhanvir Singh Rana Department of Physics Indian Institute of Science Education and Research (IISER) Bhopal

2 Terahertz spectroscopy on complex oxide films Why perform THz spectroscopy on thin films Collective dynamics by THz spectroscopy: CDW modes Charge-ordered manganites Charge-ordered rare-earth nickelates Free carrier dynamics in Rare earth nickelates Effect of crystal symmetry, disorder and epitaxial strain Non-Fermi Liquid behavior in systems sans I-M transition DC and THz frequency conductivity I-M transition This talk is based on: Terahertz spectroscopy in complex oxides Eswara et al, Phys Rev Mater (2018) R. Rana et al, Phys Rev B (2018) Eswara et al, Phys Rev B (2017) S. Das et al, Phys Rev B (2017) R. Rana et al, Phys Rev B (2013) R. Rana et al, Appl. Phys. Lett. (2013) Pandey et al, Appl. Phys. Lett. (2013) Eswara et al, JPCM (2017) Santhosh et al, J Phys D (2017) S. Das et al, JPCM (2016)

3 THz spectroscopy in Condensed Matter THz-gap! 1 THz corresponds to 33.3 cm -1, 4 mev, 300 µm, 50 K

4 Importance of THz spectroscopy in complex oxides Superconductivity, I-M transition, CMR, Exotic magnetism, Charge-ordering, Electric order, Multiferroic order, etc. Zhang and Averitt, Ann Rev (2012) Energy range of 1-20 mev: charge-dynamics and collective excitations Interaction of THz radiation with materials charge density waves, superconductivity gap, electromagnons, phonon modes, etc. Probes thermally inaccessible phases by time resolved time domain spectroscopy

5 THz spectroscopy on thin films Allows to probe all type of complex oxides by optimizing film thickness Epitaxial strain can tune a range of physical properties and THz dynamics Dimensionality control Possible to achieve single crystal quality with ease

6 Terahertz time-domain spectroscopy of thin films Ti: Sapphire femtosecond laser QWP-quarter wave plate, HWP is half wave plate, PBS is Polarization beam splitter Mirror 800nm, 100 fs 82MHz Control electronics + Lock-in amplifier Current amplifier Mirror Mirror Stepping motor Lens Detector (LT-GaAs photoconductive anteena) Optical delay line Cryostat Sample Mirror QWP HWP Mirror Polarized Beam Splitter Mirror Mirror Lens Emitter (LT-GaAs photoconductive anteena) Si hemisperical lens 1 THz corresponds to 33.3 cm -1, 4.1 mev, 300 µm, 50 K

7 Collective charge dynamics by THz spectroscopy: CDW modes Charge-ordered manganites Charge-ordered rare-earth nickelates

8 THz excitation of CDW in complex oxides: Manganites and Nickelates CDW in ABO 3 complex oxides Kida and Tonouchi, Phys Rev. B (2002) G. Gruner, Rev Mod Phys (1988) Cox et al, Nature Materials (2008) Rana et al, PRB (2013), APL (2012) Can THz spectroscopy probe such low energy CDW modes?

9 Charge-density-waves in charge-ordered Pr 0.5 Sr 0.5 MnO 3 THz conductivity of Pr 0.5 Sr 0.5 MnO 3 films: CDW collective mode in (110) films 0.5 Kajimoto et al.,, PRB (2002) Nd 0.5 Sr 0.5 MnO 3 :Effect of ferromagnetic order Rakesh Rana et al, JPCM (2013); J spect. dyn. (2012) Eu 1-x Sr x MnO 3 (x = 0.5, 0.58): Effect of spin-glass disorder Parul Pandey et al, App Phys Lett (2012) Rakesh Rana et al, Appl Phys Lett (2012) Rakesh Rana et al, Phys Rev B (2013)

10 Rakesh Rana et al, Phys Rev B (2013) Rakesh Rana et al, Appl Phys Lett (2013) Charge-density-wave excitations in R 0.5 Sr 0.5 MnO 3 Pr 0.5 Sr 0.5 MnO 3 : Anisotropic charge-order Gradual crossover from CDW mode along (001) plane to Drude behavior along (1-10) plane. Resolved that i) charge orders along (001) plane and iii) CDW is of generic nature

11 T (K) ρ~ T 2 Perovskite Nickelates: Phase diagram RNiO 3 (R = rare earth ion) NdNiO 3 : E-type AFM order and I-M transition at ~200 K ρ~ T n Tuning parameter (Epitaxial strain) NdNiO 3 Below T N =T IM ~ 205 K Complex AFM ordering E-type with spins NdNiO 3 Insulator metal transition tuning by chemical doping, strain, magnetic field, etc. Landau theory: AFM ground state to be a nested spin density wave which induces charge order as secondary effect. Charge order proportional to degree of orthorhombic distortion. Charge order is absent for tetragonal /cubic symmetry. [Lee et al. PRL (2011)]

12 NdNiO 3 (100), (110) and (111) thin films on LaAlO 3 substrate Reciprocal space mapping (100) film- 35nm (110) film- 240 nm Sample In-plane lattice In-plane lattice Out-of-plane lattice Orthorhombicity constant a (nm) constant b (nm) constant c (nm) (b/a) (100) 240nm (110) 240nm (111) 240nm (100) 35nm (110) 35nm (111) 35nm

13 THz conductivity of NdNiO 3 Collective excitations in orthorhombic NdNiO 3 (100), (110) and (111) films (100) (110) (111) Dielectric dispersion

14 (b) (100) 240nm ) ( ) ( n n n n d P d S CDW collective excitations versus Drude behavior (110) 240nm (Drude + Lorentz) 2 o AT Scattering Rate

15 R. Rana et al, Phys Rev B (2018) Structural control of CDW in NdNiO 3 The CDW mode manifest in the orthorhombic distorted structure only, as predicted by Landau theory. The strength of the mode scales with the orthorhombic distortion.

16 Free charge carrier dynamics study in Rare earth nickelates Effect of crystal symmetry, disorder and epitaxial strain Non-Fermi Liquid behavior

17 Epitaxial strain tuned charge dynamics in PrNiO 3 films Synthesis of coherently strained epitaxial PrNiO 3 thin films Compressive epitaxial films on LaAlO 3 (100) and (110) substrates Tensile epitaxial films on NdGaO 3 (100) (001) (110) and (111) substrates

18 THz conductivity models for free carrier dynamics Drude Model Drude carrier dynamics in PrNiO 3 /LaAlO 3 films Drude-Smith Model THz carrier dynamics are strain dependent Compressive films exhibit Drude-type dynamics Tensile and orthorhombic films are dominated by Drude-Smith type carrier dynamics

19 Drude-smith dynamics in PrNiO 3 /NdGaO 3 films: Spectral weight shifts to higher frequency All the attributes of Drude-smith carrier dymanics Eswara et al, Phys. Rev. B (2017)

20 Drude-smith dynamics in PrNiO 3 /NdGaO 3 films: Drude peak shifts to higher frequencies The disorder parameter scales with O 2 deficiency in the films Suggests O 2 vacancies or phase-separation the cause of back-scattering Eswara et al, Phys. Rev. B (2017)

21 Effect of cation disorder: (LaEu)NiO 3 and NdNiO 3 thin films Average A-cation radius (<r A >= Å ) of (La 0.5 Eu 0.5 )NiO 3 (LENO) is same as well-studied NdNiO 3 (NNO) Vegard s rule a p (LENO) = (a p (LaNiO 3 ) + a p (EuNiO 3 ))/2 ~ Å (a p (NdNiO 3 ) ) A-site cation size variance v2 = <r A2 >- <r A > 2 = Å 2

22 Effect of disorder in (LaEu)NiO 3 : Spectral weight transfer LENO/LAO_30nm LENO/NGO_30nm Drude conductivity Broad 1 peak at = 0 indicates the presence of disorder in LENO/LAO film Relaxation time t = 1/ decreases slightly with an increase in temperature Drude-Smith conductivity 1 peak at = 0 shifted to higher THz frequency No CDW mode below T IM S. Das et al, Phys Rev B (2017)

23 RNiO 3 phase diagram and quantum criticality T (K) Nickelates: Tunable insulator-metal transitions, quantum phase transitions, etc. Motivation: To suppress Mott insulating phase by quantum tuning To explore hidden non-fermi liquid phases in proximity to quantum critical point (QCP) To explore the signatures of quantum critical behavior, if any

24 T (K) Epitaxial strain tuned quantum phase transition Tensile compressive 100 (mcm) 10 1 PNO/NGO (001) PNO/LAO (100) xx =1.33% xx = -0.65% T 2 PrNiO 3 NdGaO T T (K) PrNiO 3 LaAlO 3 ρ~ T n ρ~ T 2 According to parallel resistivity model 1 = ρ(t) ρ 0 +AT n ρ SAT 10 0 Tuning parameter (Epitaxial strain) By tuning epitaxial strain PNO/NGO PNO/LAO film FL [n= 2.0] NFL [1.62 5/3] crossover

25 40 Extended Drude analysis: NFL-like behavior in PNO/LAO film 1 ( cm) K 60 K 120 K 240 K 300 K Γ(ω) = A[(ħω) 2 +(2πk B T) 2 ] (a) ( cm) (z ) PNO/LAO (100) (b) 20 0 ( cm) (z ) ρ 1 (ω) = Γ(ω) ε 0 ω p 2 = [ ρ(ω) = ρ 1 (ω) + iρ 2 (ω) = 1/ σ (ω)] σ 1 (ω) σ 1 (ω) 2 + σ 2 (ω) 2 ρ 2 ω = σ 2 (ω) σ 1 (ω) 2 +σ 2 (ω) 2 The non-linear behavior in ν 2 and ν domain implying a NFL-like behavior 0 PNO/LAO (100) (b) (z) Eswara et al, Phys. Rev. Materials (2018)

26 Frequency dependent I-M transition in Rare earth nickelates DC and THz frequency conductivity

27 DC to THz frequency conductivity: Insulator-metal transition S Das et al (in Progress)

28 Similarity of DC and THz conductivity in metallic nickelates For metallic systems, the DC and THz conductivities are nearly same

29 Frequency dependent I-M transition: Role of competing interactions Phase-separated nano-scale insulating cluster formed in threshold percolative metallic path below MIT for cooling and heating protocols.

30 Summary THz spectroscopy is indispensable to unravel complex phenomena in complex systems. THz time domain spectroscopy can probe the low energy modes such as THz CDW collective excitations in electronically ordered phases. Distinct THz spectral features due to free carrier dynamics, as induced by different types of disorder in nickelates, underlines the importance of this technique over the dc conductivity. Frequency dependent Insulator-metal transition, induced by subtle interplay of two competing phases, is a novel observation and first of its type. MIT of nickelates has the potential to design efficient transmission THz modulators.

31 Future directions and open questions How to distinguish between electronic and magnetic origin of excitations? Magnetic control of low energy THz excitations Magnetic field dependent THz spectroscopy Ideal to probe emerging systems governed by energetic of Spin-Orbit coupling Non equilibrium charge and spin dynamics: Time resolved THz spectroscopy

32 Acknowledgements Group members: Amit Khare (Inspire faculty) PhD completed: Rakesh Rana, Parul Pandey, P E. Phanindra PhD ongoing: Sarmishta Das, K Santhosh Kumar, S. Sardar, G.L. Prajapati, Rahul, Anagha P, Monu BS-MS students: S. Tripathi, Kailash Dhaker, H. Vashishta, Kanchan Yadav, Piyush Agarwal, P. Arjun, P. Anagha, Monu Kinha, Sidharth Sharma Collaborators: S. S. Prabhu, TIFR Mumbai M. Tonouchi, Osaka University, Japan Krushna Mavani, IIT Indore Funding: IISER Bhopal DST-SERB, New Delhi DST-Nanomission, New Delhi DST-FIST, New Delhi

33 Structure and strain analysis of (LaEu)NiO3 films Reflectivity and reciprocal space mapping Epitaxial strain on LENO for LAO, NGO and STO substrates is -0.23%, 1.4% and 2.6%, respectively. Data suggests films are high quality and coherently strained

34 Cation disorder induced suppression of MIT: NdNiO 3 versus (LaEu)NiO 3 S. Das et al, Phys Rev B (2017)