Electron Spin Resonance and Quantum Dynamics. Masaki Oshikawa (ISSP, University of Tokyo)

Transcription

1 Electron Spin Resonance and Quantum Dynamics Masaki Oshikawa (ISSP, University of Tokyo)

2 Electron Spin Resonance (ESR) E-M wave electron spins H measure the absorption intensity

3 Characteristic of ESR single spin problem interaction among spins: changes the lineshape? In fact, the lineshape remains exactly the same if the interaction is isotropic total spin

4 Why no change? Wavelength of the oscillating field >> lattice spacing etc. ESR measures (motion of total spin! ) because No change in eq. of motion!

5 Effect of anisotropy (small) anisotropy In the presence of anisotropy, the lineshape does change. eg. shift and width ESR is a unique probe which is sensitive to anisotropies! e.g.) 0.1% anisotropy in Heisenberg exchange could be detected experimentally with ESR

6 Pros and cons of ESR ESR can measure only q» 0 But.. very precise spectra can be obtained with a relatively small and inexpensive apparatus The real problem: interpretation of the data requires a reliable theory, which is often difficult

7 ESR as a fundamental problem ESR is a fascinating problem for theorists Fundamental theories on magnetic resonance: ~ 1960 s J. H. van Vleck, P.W. Anderson (Nobel Prize 1977) (Nobel Prize 1977) R. Kubo K. Tomita (Boltzmann Medal 1977) H. Mori K. Kawasaki (Boltzmann Medal 2001) R. Kubo P.W. Anderson

8 origin of the general linear response theory

9 What should we do? Restrict ourselves to linear response regime: just need to calculate dynamical susceptibility Anisotropy is often small: formulate a perturbation theory in the anisotropy This sounds very simple, but not quite! cf.) Prof. Kitahara s remark this morning

10 10/32 Difficulty in perturbation theory (I) If the (isotropic) interaction is strong (i.e. exchange interaction J not small compared to H, T ) 0-th order Hamiltonian is already nontrivial (although the ESR spectrum appears trivial ) ESR probes a collective motion of strongly interacting spins, not a single spin

11 Difficulty in perturbation theory (II) 0th true 1st 2 nd width H H shift Any (finite) order of the perturbation series in is not sufficient.. We need to sum over infinite series in some way

12 Phenomenological Theory (I) Bloch Equation longitudinal/transverse relaxation time phenomenological description of irreversibility

13 Phenomenological Theory (II) Solving the Bloch eq. up to the first order in h (linear response regime) ESR spectrum becomes Lorentzian, with the width The ESR width reflects the irreversibility! How to derive the width microscopically?

14 Past Theories Kubo-Tomita (1954), Mori-Kawasaki (1962) etc. Two problems in these standard theories 1. based on several nontrivial assumptions: the fundamental assumptions could break down in some cases. 2. evaluation of correlation functions are done within classical or high-temperature approximations. not valid with strong quantum fluctuations

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16 S=1/2 Heisenberg AF chain at low temperature: extreme limit of strong quantum fluctuation most difficult problem to handle, with the previous approaches However, we can formulate a new approach to ESR based on field theory (bosonization) M.O. and I. Affleck,

17 Bosonization S=1/2 (isotropic) Heisenberg AF chain c=1 free boson (Tomonaga-Luttinger liquid) low-temperature, low-energy ESR spectra is given by, within the field theory, -- Here I skip the subtle derivation! -- [Green s function of the fundamental boson]

18 ESR spectrum in bosonization Isotropic Heisenberg chain = free boson just reproduces the known result, but now the starting point is the free theory! (solving Difficulty I) Anisotropy: often gives rise to interaction Diagrammatic perturbation theory can be formulated (Feynman-Dyson) self-energy summation solves Difficulty II

19 Self-energy formulation = Shift: Width:

20 20/32 Physical Interpretation ESR absorption = creation of a boson (quantized spin fluctuation) with momentum H In the isotropic system, the boson has an infinite lifetime, and the energy is exactly H In an anisotropic system, the bosons are interacting and thus has a finite lifetime Uncertainty relation uncertainty in the energy width in the spectrum

21 Application: staggered field bosonization The self-energy can be exactly given in the lowest order of perturbation O(h 2 ) = -

22 Result for the staggered field Shift Width (up to the leading log) Diverging shift/width at the low temperature ---- is it observable?

23 Cu benzoate very good 1D Heisenberg AF chain with J» 18 K (Neel temperature < 20 mk!) studied extensively in s by Date group but with some strange features which were not explained. ( including ESR! ) 1997: neutron scattering under magnetic field (Dender et al.) found a field-induced gap an effect of the effective staggered field! (M.O. and I. Affleck, 1997)

24 Crystal structure of Cu benzoate chain alignment of molecules surrounding Cu 2+ : alternating along the chain staggered g-tensor, staggered Dzyaloshinskii-Moriya int. effective staggered field is generated depends on field direction

25 1972(!) H-dependent divergence at low T H-independent part (linear in T at low T) due to exchange anisotropy

26 ESR linewidth in Cu benzoate H / resonance frequency data from Okuda et al. (1972) [H-dep.part]

27 Angular dependence assumed a DM vector which fits other expt. as well

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29 Exchange anisotropy / dipolar int. If the crystal symmetry does not allow the staggered field, the most dominant effects on ESR come from exchange anisotropy / dipolar interaction e.g. width from our theory:

30 Comparison with experiments

31 Bosonization approach does work for ESR, but remember that it applies only to 1D systems in the low-energy limit! Kubo-Tomita theory seems to work in some cases, but its range of validity is not established.

32 Conclusion ESR provides challenging and fundamental problems in statistical physics Many open problems We need to know the detailed nature of quantum dynamics of the system in question

33 Following materials are not covered in the talk but added for your information.

34 What happens at lower T? The lowest order result diverges as T! 0 i.e. the perturbation theory breaks down at sufficiently low T, even for a small h Can we say something? The low-energy effective theory is asymptotic freedom integrable sine-gordon QFT!

35 At T=0 : excitations from G.S. the elementary excitations are soliton/antisoliton/1 st breather : same mass M and 2 nd breather of mass Exact sine-gordon formfactor (Karowski-Weisz) Remember: q=h for ESR spectrum from QFT 1 st breather dominant (small incoherent part)

36 Prediction on ESR at T=0 incoherent part (small) Huh, didn t I say the linewidth diverges as T is lowered?

37 1978(!) original paramagnetic peak» H new peak narrowed as T! 0 sg breather

38 Testing the sine-gordon prediction Used the same set of the parameters as in the perturbative regime

39 finite-t dynamical correlation function in the sg QFT is seen here!!

40 T-dependence of the width crossover at T» M

41 Effect of the bond alternation bosonization interaction among bosons: finite lifetime of the boson = finite ESR linewidth? wrong! In fact, the ESR lineshape remains exactly the same, because it is SU(2) symmetric This illustrates the subtlety in the ESR theory

42 Open problems: coupled chains An interesting extension of the 1D theory would be weakly coupled chains. However, it is rather difficult because we have to correctly handle the subtle SU(2) symmetry

43 Problem example (Y. Ajiro) coupled chains with different g-factors interchain coupling

44 What about higher T? Field theory only works at low enough T For higher T, we cannot rely on field theory and there is no diagrammatic perturbation theory available in general. Can we still say something?

45 Focusing on the shift only Forget the full lineshape! -- to avoid the infinite summation Kanamori-Tachiki (1962), Nagata-Tazuke (1972) single mode approximation shift Nagata-Tazuke then evaluated this formula in the classical & weak field limit. But the validity of the single mode approximation (and thus of the shift formula) was unclear

46 New derivation Define the shift by the first moment expand in Kanamori-Tachiki formula is generally exact in the first order, but NOT in second and higher orders!

47 Second-order formula Y.Maeda-M.O. J. Phys. Soc. Jpn. 74, 283 (2005)

48 Exchange anisotropy Antisymmetric exchange (DM interaction) gives the shift only in the second order So we only consider the symmetric exchange anisotropy between nearest neighbors (diagonalized by taking principal axes)

49 First-order shift c H b a to be evaluated for S=1/2 Heisenberg AF chain in the field H//z

50 Exact evaluation of Y(T,H) Consider a fictitious XXZ chain in the field H Free energy (per site) F is known exactly for arbitrary H,, T by the Quantum Transfer Matrix technique. desired term in Y!

51 Exact solution C Re x

52 g-shift ESR frequency shift is often proportional to the applied field H (e.g. in Nagata-Tazuke) It is thus customary to discuss the shift in terms of effective g-factor g shift

53 Result agree with classical limit (Nagata-Tazuke) at high T strong H-dependence at low T!

54 Low-temperature expansion For H << T << J,

55 Comparison with field theory Field theory (M.O.-Affleck) gave the leading log consistent with the present result! leading log is dominant only in extremely low temperature. in realistic temperature regime, subleading corrections found here are important!

56 Comparison with experiments We want pure S=1/2 Heisenberg AF chain without the staggered field effect KCuF 3, CuGeO 3, NaV 2 O 5.. (no significant T-dependence in shift?? (why?)) LiCuVO 4 another S=1/2 Heisenberg chain Vasil ev et al. (2001) von Nidda et al. (2002)

57 ESR shift in LiCuVO 4 theory vs. experiment

58 crossover to high temperatures conjecture for the linewidth (field theory) staggered field (Kubo-Tomita) (crossover temperature)

59 crossover of linewidth exchange anisotropy / dipolar interaction (field theory) (Kubo-Tomita) (crossover temperature)

60 Kubo-Tomita failure at high T (staggered) Dzyaloshinskii-Moriya interaction Kubo-Tomita theory linewidth in the high T limit: typically, different by factor 100! However, the DM interaction can be eliminated by an exact transformation to give exchange anisotropy and staggered field. Kubo-Tomita applied after the transformation: linewidth in the high T limit: correct answer? (or even smaller)