High Field EPR at the National High. Johan van Tol. Magnetic Field Lab

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1 High Field EPR at the National High Johan van Tol Magnetic Field Lab

2 Overview EPR Introduction High Field CW EPR typical examples ENDOR Pulsed EPR Relaxation rates Qubits Relaxation at high fields Outlook

3 It s all the same. ESR - Electron Spin Resonance EPR - Electron Paramagnetic Resonance FMR Ferromagnetic Resonance AFMR Antiferromagnetic Resonance EMR - Electron Magnetic Resonance

4 Which systems have electron spin? Any system with unpaired electrons Organic radicals Usually involves aromatic systems Some molecules with degenerate p-orbitals Oxygen Transition metal ions with partially-filled d-orbitals Fe3+, Mn2+, Cr3+, etc. Rare earths with partially filled f-orbitals Gd3+, Tb3+, Eu2+, etc. Free or bound electrons (or holes) in semiconductors and metals, or bound to a surface. p- or n-doped Si, GaAs Electrons on helium films

5 The EPR phenomenon Energy H = ( l + g s ) B + ζl i i Zeeman hν e Magnetic Field (T) i m S = +1/2 m S = -1/2 i s Spin-orbit coupling i Anisotropy induced by Spin-Orbit Coupling Zeeman splitting (GHz) B//z, g z = 2.00 B//x, g x = Magnetic field (tesla)

6 Measurement principle Typically Wavelength mm Microwave resonator: Q = Pcav/Pin = ω/δω~1000 Frequency is fixed Field Modulation:

7 At High Field Modulation or no modulation When lines are much broader than modulation allows Resonator or no resonator Sample size in resonator scales with wavelength ~λ/10 Better concentration sensitivity with larger samples

8 The effective spin hamiltonian Η µ B g S S D S S A I γ I = B + + j n j + k + k Tdip j k Zeeman Hyperfine structure Fine Structure Nuclear Zeeman Dipolar B JS S Exchange S S Frequency (GHz) S= Magnetic Field (T)

9 Hyperfine Structure Isotropic (Fermi contact term) Due to spin density on the nucleus. Only s-orbitals have spin density on the nucleus Core polarization: spin density in p- d- or f- orbitals polarizes filled s-orbitals. Anisotropic part: dipolar contributions Nitroxide free radical attached to a protein S=1/2 I = 1 ( 14 N) Intensity (mv) Here the largest part of spin density is in a p-orbital in the N-O bond. Partially delocalized on the rest of the molecule: Unresolved proton hyperfine Field (T)

10 Why at High Field/Frequency? High absolute sensitivity (~10 10 spins at 10 GHz, ~10 6 spins at 1000 GHz) (->smaller samples) Some high-spin systems cannot be accessed at low frequencies Higher Zeeman resolution (and orientational selection) High-Field phases High electron spin polarization (hν >> kt) Faster timescale

11 Tunable-frequency, high-field EPR of octahedral Fe(II): Bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II); First EPR characterization of high-spin Fe(II) in octahedral geometry. Magnetic Field (Tesla) Frequency (GHz) a > 1 s > 0'> 1 a > 0'> 1 s > Energy (cm -1 ) Field vs. quantum energy dependence of EPR resonances in Fe(btz). Ozarowski, A.; Zvyagin, S. A.; Reiff, W. M.; Telser, J.; Brunel, L. C.; Krzystek, J., J. Am. Chem. Soc., 2004, 126,

12 System with phase transitions induced by the field The ESR frequency-field dependence (circles) and the line-width, B (squares) in CuGeO %Si, B // a, T = 4.2 K. The lines are guides to eye.

Influence of mutations in photosynthetic reaction centers The photosynthetic reaction occurs along the A-branch, in spite of the high symmetry of the complex.

13 Influence of mutations in photosynthetic reaction centers The photosynthetic reaction occurs along the A-branch, in spite of the high symmetry of the complex. A central role here is played by the (a)symmetry of the spin density on the primary donor chlorophyll dimer. The question to be answered here is how the protein ligands of the dimer influence the spin density distribution on the dimer.

EPR intensity g-factor resolution WT H676Q H656Q HHQQ The influence of the replacement of hystidine ligands in positions 676 and 656 by glutamine and threonine-739 by alanine

14 EPR intensity g-factor resolution WT H676Q H656Q HHQQ The influence of the replacement of hystidine ligands in positions 676 and 656 by glutamine and threonine-739 by alanine on the 330 GHz EPR spectra. The significant shifts in g zz value indicate deviations from planarity of the molecule. T739A g-value K. Redding UA

15 LaCaMnO 3 on different substrates 240 GHz, 295 K SrTiO 3 Si EPR intensity NdGaO 3 MgO LaAlO Field (T)

16 Possible pitfalls Saturation If relaxation is slow, spin system can be driven out of equilibrium Impurities Perhaps what you are measuring is only related to impurities Interferences When absorption is strong, index of refraction and wavelength inside sample changes when going through resonance. If sample size ~ λ interferences.

17 Pulsed EPR M B 0 -ω 0 /γ B 1 Relaxation parameters T 1 Spin-lattice relaxation time T 2 Spin-memory time, spin-spin relaxation time T 2 * Free-induction decay time (inhomogeneous linewidth) Pulselength must be short with respect to the relaxation times!

18 Why pulsed? Relaxation measurements ( T 1, T 2 ) Dynamics, spin-spin interactions Electron-electron spin couplings Distance measurements in proteins in the 1.5 to 6 nm range Electron nuclear couplings Hyperfine interactions (ESEEM, HYSCORE, pulsed ENDOR) Spin manipulation Quantum computing etc.

19 Example Information hidden under a single EPR line revealed by HYSCORE (Fe(CN)63in NaCl) 8 (MH z) Fre que ncy Freque 0 ncy (M 2 Hz)

20 DEER or pulsed ELDOR _π π 2 t 1 ω 1 π t 1 t 2 t 2 π ω 1 ω 1 cos (ω ee t) C65 t Time π ω 2 C135 Time g yy g zz Site-directed spin labeling Protein structure Changes in structure g xx A zz Field / gauss

21 C65 R C89 ν dip 2.2 MHz R = 28 Å Time / µs Distance / nm C65 R ν dip 0.5 MHz R = 46 Å (46 ± 2) Å C Time / µs Distance / nm

22 Electron Magnetic Resonance X-band 0.3 T 10 GHz Spectral domain and time domain (pulsed) W-band 3 T 95 GHz D-band 5T 140 GHz Spectral domain only (CW) / 15 T 330 / 440 GHz NHMFL KECK 25 T BWO GHz FIR 250 2,700 GHz B(T) O 1OO commercial homebuilt Single Mode Waveguide Quasi Optics - Overmoded Corrugated Waveguide

23 In superconducting magnets: Free space transport and Quasi-Optical Techniques Jurek Krzystek GHz EPR in 25 Tesla magnet (homogeneity 10-5 /cm 3 ) BWO sources.

24 Instrumentation Home-built Quasi-optic Multifrequency 120, 240, 336 GHz CW EPR/ENDOR Pulsed EPR/ENDOR Rev. Sci. Instrum. 79, (2008)

25 Transition ion system K 3 NbO 8 :Cr 5+ Suggested standard for hf- EPR, simple 3d 1 system. Transition ion qubit with S=1/2 and I=0 or I=3/2 Phys. Rev. Lett. 99, (2007)

26 Pulsed ENDOR at 240 GHz Mims ENDOR π/2 π/2 π/2 Stimulated echo µ-wave RF Hyperfine and quadrupole interaction Electronic (spin) wavefunction. Nellutla et al. PRB in press

27 N centers in SiC High power, high temperature Polymorphisms Mostly hexagonal structure 4H 6H 2H-SiC

28 High-Frequency EPR and ENDOR of N centers in SiC 240 GHz 60 K N hex N cub B // c 4H B c 6H Field (T) Field (T) July 2008 Rocky Mountain Conference, Breckenridge

Pulsed ENDOR 88 89 90 91 92 93 94 95 91.50 91.75 92.00 92.25 92.50 92.75 93.

29 Pulsed ENDOR C (upper scale) a 3 a 2 29 Si (lower scale) ENDOR Frequency (MHz) a 1

30 Electron Spin Relaxation T 1 Relaxation Longitudinal relaxation. Involves a change in magnetization. Involves energy exchange with surroundings Population transfer between spin states. T 2 Relaxation Transverse relaxation (with respect to field) Phase memory time No energy exchange Change in local field

31 Spin Lattice Relaxation T 1 Indirect processes e - E/kT, T 5, T 7 frequency independent 2 phonon processes Direct process frequency dependent ω 2 T or ω 2 B 0 T Single phonon process

32 Cr 5+ in K 3 NbO 8 T 2 frequency independent, T 1 ~ 250 times faster at 240 GHz as opposed to 10 GHz at low temperatures

33 Phosphorus in Si T 1 (s) 10 1 T 1e 9.7 GHz T 1e 95 GHz T 2e 9.7 GHz T 2e 95 GHz Fit to exponential 240 GHz 130 K = 11.2 mev /T (K -1 ) T 1 at low temperatures is about 3 orders of magnitude shorter at 240 GHz as compared to X-band Easier measurements Faster system reset Quantum computing can be faster

34 Relaxation: Spin-memory time T 2 Nitrogen related centers in diamond Takahashi et al, PRL

35 What about T 1 here? T 1 stays very long in this system, fitted by JT-induced relaxation for Nitrogen Relatively deep donors with g-values very close to g e.

36 Flip-flop rate reduction at high field and low T Strong reduction of relaxation due to (B-spin) flip-flops. 1/T 2 ~ Γ hyp + c*p up *P down (Kutter et al. PRL 74, 2925, 1995)

37 T 2 : Fe 8 S=10 Molecular Magnet Note that this is a very concentrated system. T2 is dominated by electron-electron dipolar interactions

38 Coherence time in Fe8

center is involved In order to measure a current we use light

39 Electrical detection of Magnetic Resonance (Read out) Electrical detection can be very sensitive At low frequencies a P b (surface) center is involved In order to measure a current we use light excitation to create carriers T 1 is shortened, T 2 more or less unchanged

40 EDMR spectra Up to 10% current changes on phosphorus resonance McCamey et al., PRB 2008

41 Mechanism

42 Coherence? At low frequency the electrically detected spin coherence laster for ~ 2 µs due to fast recombination of electron-hole pairs. At high frequency the main mechanism is dominated by spin traps -> longer coherence times? Morley et al (Arxiv)

43 Light Induced Nuclear Polarization

44 Spin dynamics at High Field High Power -> short pulses -> High time resolution HIPER 1 kw at 94 GHz with 1 ns deadtime Future: Higher frequencies (100 GHz 3 THz)

45 UCSB-FEL Rocky Mountain Conference, Breckenridge July 2008 Injection locking (phase-locking) has now been achieved. Collaboration has led to a (1.7M) grant from The Keck Foundation to UCSB and the NSF (1.2 M) for fast pulsed EPR at 240 GHz, in strong collaboration with the NHMFL

46 Electrostatic FEL Rocky Mountain Conference, Breckenridge Cutting pulses

47 First results with FEL at UCSB

48 Acknowledgements S. Nellutla I Chiorescu N Dalal M.E. Zvanut G. Morley A. Ardavan A. Biggs K. Porfyrakis D. McCamey C Boehme S. Takahashi M. Sherwin D. Awschalom D Hendrickson

Shallow Donors in Silicon as Electron and Nuclear Spin Qubits Johan van Tol National High Magnetic Field Lab Florida State University

Shallow Donors in Silicon as Electron and Nuclear Spin Qubits Johan van Tol National High Magnetic Field Lab Florida State University Overview Electronics The end of Moore s law? Quantum computing Spin