Ultrafast X-ray Studies of Correlated Materials: Science Challenges and Opportunities

Transcription

1 Ultrafast X-ray Studies of Correlated Materials: Science Challenges and Opportunities Lawrence Berkeley National Laboratory Robert Schoenlein Materials Sciences Division - Chemical Sciences Division Ultrafast X-ray Sciences Laboratory Next Generation Light Source LCLS-II New Instruments Workshops March 19-22, 2012

2 Ultrafast Dynamics in Complex Materials - Beyond Bloch How do the properties of matter emerge from the: correlated motion of electrons, and coupled atomic and electronic structure? - beyond single-electron band structure models, Bloch, Fermi Liquid Theory complex materials exhibiting strong correlation among charges, and between charge, spin, orbit, and lattice Oxides of Transition Metals (Cu, Mn, Ni, V ) T lifetime (E-E F ) -1/2 Understand the Interplay between Atomic and Electronic Structure - Valence electronic structure energy levels, charge distribution, bonding, spin - Atomic structure coordination, atomic arrangements, bond distances

3 Fundamental Time Scales in Condensed Matter Atomic Structural Dynamics atomic vibrational period: T vib = 2p(k/m) -1/2 ~ 100 fs k~ev/a 2 m~10-25 Kg ultrafast chemical reactions ultrafast phase transitions Electronic Structural Dynamics electron-phonon interaction ~ 1 ps e-e scattering ~10 fs e - correlation time ~100 attoseconds (a/v Fermi ) bond dynamics, valence charge flow charge transfer ultrafast biological processes O Fe N electronic phase transitions - correlated electron systems N Ultrafast Measurements: - separate correlated phenomena in the time domain - direct observations of the underlying correlations as they develop

4 Ultrafast X-rays Powerful tool for understanding correlated materials Ultrafast x-rays will probe correlations among charges and between electronic and atomic structure On fundamental (mev) energy scales of low-energy excitations, with full momentum resolution With sensitivity to spin and magnetic order Using tailored excitations to separate correlated phenomena in the time domain

5 Crystal structure of Manganites leads to complex phase diagram, and exotic electronic properties Temperature [K] Pr 1-x Ca x MnO PI T CO 200 COI 150 T C T N 100 T N AFI Pr 1-x Ca x MnO CI FI x CAFI Spin-ordered (SO) phases Colossal Magneto- Resistance (CMR) resistivity ~1/B N Insulator-Metal Phase transitions driven by applied magnetic field (CMR), and by ultrafast optical excitation (photo-doping): F th ~4 mj/cm 2

6 Vibrationally Driven I-M Transition in a Manganite M. Rini, et al., Nature, 2007 mid-ir mm 1mJ, 200 fs Pr 1-x Ca x MnO 3 Mn-O bend Mn-O stretch O Mn 3+ Mn 4+ d Pr O 2 d Mn O THz vibrational control of correlated-electron phases targeting specific vibrational modes - Mn-O stretch Ultrafast I-M phase transition - electronic ground state x10 4 resistivity change Ultrafast X-ray techniques relevant for a broad range of complex materials (organics, multiferroics, novel superconductors..) Tolerance Factor Mn-Mn hopping rate charge de-localization Scientific Questions and Challenges Electronic Structure: Dynamics of charge localization/delocalization? ultrafast XAS Mn-3d/O-2p hybridization Dynamics of charge/orbital/spin ordering? ultrafast resonant x-ray diffraction Magnetic nature of the metallic phase ferromagnetic? ultrafast x-ray dichroism, magnetic scattering

7 Temperature [K] Charge/Orbital Ordering in Manganites Time-resolved resonant x-ray diffraction Pr 1-x Ca x MnO 3 spin down spin up PI T CO COI Mn 4+ Mn T N T C T N AFI Tokura et al., PRB, CI FI CAFI x Pr 0.5 Ca 0.5 MnO 3 CO/OO/SO charge localization FM charge delocalization Order dynamics (formation/melting) Underpins the insulator to metal transition 65 K Mn L-edge (¼ ¼ 0)

8 spin ordering Charge/Orbit/Spin Ordering in Manganites Resonant X-ray Diffraction Pr 0.7 Ca 0.3 MnO 3 Mn L-edge (¼ ¼ 0) Scattering spectra differ dramatically from XAS Different azimuthal angular dependence Strong spin ordering component (compared with 50% doping) Pr 0.5 Ca 0.5 MnO 3 S. Zhou et al. Phys. Rev. Lett., 106, (2011) ALS Beamline 8.0

9 Charge/Orbital Ordering in Manganites Pr 0.5 Ca 0.5 MnO 3 Temperature [K] Pr 1-x Ca x MnO T N PI T C T CO T N COI AFI 50 CI FI CAFI x

10 Charge/Orbital Ordering in Manganites Pr 0.7 Ca 0.3 MnO 3 T CA T N OO T CO/OO SO stabilized only below T CA OO is weak disrupted by charge disproportionation SO

11 Resonant X-ray scattering selectively probes the spin-ordered (SO) phase PCMO Pr 0.7 Ca 0.3 MnO 3 f= ps delay X-ray pulses follow the dynamics of the spin-ordered phase: melting and recovery

12 A new microscopic picture of SO and its relation to the insulator/metal phase transition emerges FM FM FM FM F < 4 mj/cm 2 F > 4 mj/cm 2 Insulator: charge dynamics are along 1D spin-ordered chains Metal: charge dynamics are 3D, i.e. between chains, due to metallic domains S.Y. Zhou et al., in review

13 Non-thermal dynamics of stripe nickelates X = 0.25, Stripe RSXS directly measures the order parameters of both CO and SO. Both CO and SO are suppressed by pump laser excitations Rich information is contained in the recovery behavior. La 2-x Sr x NiO 4 LCLS - SXR W. S. Lee, Y. D. Chuang et al., Nature Comm. (in press)

14 Number of surprises CO No change of diffraction peak position and peak width. CO and SO s period and correlation unchanged. -> No topological defects are created. RSXS on CO Optical Reflectivity Dynamics of order parameters amplitude and phase can be disentangled. Order parameter s phase fluctuation is the bottleneck of the recovery. SO CO comparison Initial recovery time scale of CO and SO are comparable, despite of distinct microscopic interactions for the spin and charge. Cooperative dynamics of CO and SO due to their strong coupling effect. W. S. Lee, Y. D. Chuang et al., Nature Comm. (in press)

15 Collinear-to-Spiral Anti-Ferromagnetic Phase Transition in CuO PSI / ETHZ / LCLS / Stanford / LBNL / Oxford What is the ultimate speed of magnetic phase transitions? CuO: antiferromagnetic phase transition - Time-resolved resonant x-ray diffraction measures time t p for start of transition - Minimum lag-time of 400 fs: limited by spin dynamics S. Johnson, de Souza, Staub et al., PRL 108, (2012) LCLS - SXR Johnson (ETH Zurich), Staub, Beaud, Ingold (PSI)

16 Current and near-future projects Johnson (ETH Zurich), Staub, Beaud, Ingold (PSI) Multiferroics: dynamics of domain switching How fast can electric field control of magnetism occur in an induced multiferroic? Interaction of lattice, orbital, charge & spin orders Can we control electronic properties of correlated systems by dynamically manipulating structure? Kimura, Ann. Rev. Mater. Res. 37, 387 LCLS - SXR

17 La 0.5 Sr 1.5 MnO 4 mid-ir lattice excitation Prompt shift in magnetic- and orbital-order parameters Control of magnetism through ultrafast lattice excitation Importantly, a CCD only measures a slice in reciprocal space Extended vol. in reciprocal correlations lengths in k-space. no measurable change in in-plane correlations (along a- and b- axis) Sizeable change in scattering wavevector and line shape along the c-axis

18 RSXS Endstation Overview Detector 360o rotation in the horizontal scattering plane (2q) 90o rotation in the vertical scattering plane (g) 2 APDs + 1 thermopile (IR) (configurable) CCD can access back scattering angle ~155o Motorized six strut system for alignment Sample 360o azimuth rotation (f) 360o rotation in the horizontal scattering plane (q) 100o flip rotation (virtual axis, c) 15K (2Hr) up to 450K (limited by diode) Transferrable sample holder (max ~17mm OD mounting area) Y.-D. Chuang et al. Advanced Light Source

19 From TR RSXS to Time- & q-resolved RIXS Kaydon bearing rotates chamber + spectrometer by >30 deg Race-track bellows enables the rotation Multiple emission port covers scattering angles from ~0 to 150 deg. Manipulator and cryostat Rotary seal Incident photon beam Race-track type bellows Kaydon bearing CCD Y.-D. Chuang, Z. Hussain et al. Advanced Light Source Z.X. Shen et al. Stanford/SLAC

20 Acknowledgements S. Zhou Y. Zhu M. Langner M. Rini LBNL - Materials Sciences Y.-D. Chuang Z. Hussain E. Glover M. Hertlein LBNL- Advanced Light Source Robert Kaindl Joseph Robinson Giacomo Coslovich LBNL Materials Sciences Wei-Sheng Lee Donghui Lu Rob Moore Mariano Trigo David Reis Joshua Turner William Schlotter Oleg Krupin Z.X. Shen Stanford/SLAC Y. Tomioka JRCAT Tsukuba Y. Tokura U. Tokyo

21 Instrumentation Needs RSXS scattering chamber with control of sample orientation, software etc. High-speed, low-noise X-ray area detectors Capability for applied magnetic field >1 Tesla Time- and q-resolved RIXS (multiple q, <100 mev resolution) Soft X-rays (spanning transition-metal L-edges) Full X-ray polarization control (differential signal sensitivity <1%) <100 fs (10 fs ) temporal resolution <100 mev soft X-ray spectral resolution Hard X-rays (~1 Å resolution) q charge order Tailored laser excitation (UV-visible-THz), ~10 mj/cm 2, BW/pulse duration Future directions: Coherent stimulated Raman (stimulated scattering) XPCS spontaneous dynamics