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Spectroscopic studies of the two-dimensional magnetic insulators chromium trichloride and chromium tribromide. I
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ABSTRACTS OF ARTICLES TO BE PUB USHED IN J. PHYS. CHEM. SOUDS Vol. 27, No.4 4T chemical diffusion coefficients of electron holes in 27~2and 2 states of CrBr3 2T and the ~7’2state of CrC13 undoped Pb12 between 150 and 300°C. are examined in 2E detail. The by1 state is split field, into well2A and sublevels the trigonal with Received 21 February 1978 resolved Revised 8 June 1978 no apparent complication from vibronic coupling. A hotmagnon-assisted transition to the 2A sublevel (hWm~= 5. SPECTROSCOPIC STUDIES OF THE TWO75 cm’) is identified and its intensity vs temperature DIMENSIONAL MAGNETIC INSULATORS related to the nearest-neighbor two-spin correlation CHROMIUM TRICHLORIDE AND function, which is also used to estimate an excited-state CHROMIUM TRIBROMIDE. i exchange field of 1200kG from the shift in energy of the 2E sublevel with temperature. A single-ion intensity Victor M. Bermudez and Donald S. McClure, mechanism is also found to be important, and the resultFrick Chemical Laboratory, Princeton Univer. ing strong field dependence the temperature 2E sublevel intensity on is sity, Princeton, NJ 08540, U.S.A. exchange directionofand (below T~) The general aspects of the crystal field spectra of
analyzed in terms of 2T
4T 2T 4T 1 — 2 and 1— 1 spin—orbit mixing. Closely similar results are found for 2E thesublevel 2~2 state, including a strong dependence of the intensity on exchange field direction and temperature (below T~)and a hot-magnon-assisted transition to theis 2A sublevel (hwm~ = 48 cm’). Quantitative analysis very difficult, however, because of a complicated side-
the two-dimensional magnetic insulators CrCl3 and CrBr3 are considered in ~~‘2 detail, the broad, spin-allowed andwith 4T special emphasis on 1 bands. A fit of the point-charge model Hamiltonian to the spectra is presented, with attention to the magnitude field effects. The temperature dependenceofofthe thetrigonal ~7’2 and 4T 1 dipole strengths in the range 4.2 < T < 300 K is band structure, the probable existence of a Jahn—Teller analyzed in terms of single-ion (odd-parity trigonal field effect and uncertainty as to the rigid-lattice trigonal and odd-parity vibronic) and exchange mechanisms. A splitting. A partial interpretation of the structure, based moment analysis of the MCD spectra is performed, on Jahn—Teller interaction with one or more eg unit-cell which together with dipole strength temperature vibrations, is proposed. 4TA strong tetragonal Jahn—Teller dependence data yields a reasonable interpretation of effect is found in the 2 states in CrCI3 and CrBr3, as the spin-allowed transition mechanism. The odd-parity evidenced by a long progression in the 250 and the crystal field is found to be the most important 150cm ~ e~unit-cell vibration, respectively. For CrC!3, 4T paritybreakingmechanism in both the 47~3and 1 bands, an analysis of1the shows that EJT = andzero-phonon indicates thatlevels the excited-state Both also receive some contribution from theforvibronic 500 ±8Ocm mechanism. This process is more significant the 4T 2 exchange field is anti-ferromagnetic, isotropic and 4T state than for the 1 and may involve a slightly slightly smaller in magnitude than the ferromagnetic anharmonic low-energy vibration. Evidence for a small ground-state exchange field. Vibrationally.excited levels 4T 4T exhange contribution is found in the 1 band for both of the 2 state are examined qualitatively, aided by the systems. Factors influencing the bandshapes are analysis 4Tof the zero-phonon levels. The closely-similar examined quantitatively. The antiresonance formalism CrBr3 2 vibronic structure is also analyzed through a of Sturge er aL is applied to the anomalous lineshapes of perturbation calculation, and a good description of the certain transitions in absorption and MCD and one parsplitting, relative intensities and polarizations of the ticularly complex bandshape (that for zero-phonon levels is4Tobtained, yielding a value of UT 4T 2E—2T -÷ 2— 1 in CrCI3) is analyzed through a stat- 400cm’. Spurious 2 zero-phonon structure is found istical computation. in CrCI3 containing stacking faults, but similar complications are not found in CrBr3. Received 18 July 1977 Revised 8 June 1978 Received l8Julv 1977 Revised 8 June 1978 6. SPECTROSCOPIC STUDIES OF THE TWODIMENSIONAL MAGNETIC INSULATORS CHROMIUM TRICHLORIDE AND 7. THE SHAPE OF THE POTENTIAL ENERGY CHROMIUM TRIBROMIDE. II WELL4’OF (MIMPURITIES = Li, Na, K) IN KCI: Li~ AND Victor M. Bermudez and Donald S. McClure, KF :M Frick Chemical Laboratory, Princeton UniverJ.A. van Winsum, T. Lee, H.W. den Hartog and sity, Princeton, NJ 08540, U.S.A. A.J. Dekker, Solid State Physics Laboratory, of The splitting, spectroscopic g-factors, polarized relathe University of Groningen, I Melkweg, tive intensities and temperature dependence of the 2T 1, Groningen, The Netherlands. —
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ABSTRACTS OF ARTICLES TO BE PUB USHED IN J. PHYS. CHEM. SOUDS Vol. 27, No.4 4T chemical diffusion coefficients of electron holes in 27~2and 2 states of CrBr3 2T and the ~7’2state of CrC13 undoped Pb12 between 150 and 300°C. are examined in 2E detail. The by1 state is split field, into well2A and sublevels the trigonal with Received 21 February 1978 resolved Revised 8 June 1978 no apparent complication from vibronic coupling. A hotmagnon-assisted transition to the 2A sublevel (hWm~= 5. SPECTROSCOPIC STUDIES OF THE TWO75 cm’) is identified and its intensity vs temperature DIMENSIONAL MAGNETIC INSULATORS related to the nearest-neighbor two-spin correlation CHROMIUM TRICHLORIDE AND function, which is also used to estimate an excited-state CHROMIUM TRIBROMIDE. i exchange field of 1200kG from the shift in energy of the 2E sublevel with temperature. A single-ion intensity Victor M. Bermudez and Donald S. McClure, mechanism is also found to be important, and the resultFrick Chemical Laboratory, Princeton Univer. ing strong field dependence the temperature 2E sublevel intensity on is sity, Princeton, NJ 08540, U.S.A. exchange directionofand (below T~) The general aspects of the crystal field spectra of
analyzed in terms of 2T
4T 2T 4T 1 — 2 and 1— 1 spin—orbit mixing. Closely similar results are found for 2E thesublevel 2~2 state, including a strong dependence of the intensity on exchange field direction and temperature (below T~)and a hot-magnon-assisted transition to theis 2A sublevel (hwm~ = 48 cm’). Quantitative analysis very difficult, however, because of a complicated side-
the two-dimensional magnetic insulators CrCl3 and CrBr3 are considered in ~~‘2 detail, the broad, spin-allowed andwith 4T special emphasis on 1 bands. A fit of the point-charge model Hamiltonian to the spectra is presented, with attention to the magnitude field effects. The temperature dependenceofofthe thetrigonal ~7’2 and 4T 1 dipole strengths in the range 4.2 < T < 300 K is band structure, the probable existence of a Jahn—Teller analyzed in terms of single-ion (odd-parity trigonal field effect and uncertainty as to the rigid-lattice trigonal and odd-parity vibronic) and exchange mechanisms. A splitting. A partial interpretation of the structure, based moment analysis of the MCD spectra is performed, on Jahn—Teller interaction with one or more eg unit-cell which together with dipole strength temperature vibrations, is proposed. 4TA strong tetragonal Jahn—Teller dependence data yields a reasonable interpretation of effect is found in the 2 states in CrCI3 and CrBr3, as the spin-allowed transition mechanism. The odd-parity evidenced by a long progression in the 250 and the crystal field is found to be the most important 150cm ~ e~unit-cell vibration, respectively. For CrC!3, 4T paritybreakingmechanism in both the 47~3and 1 bands, an analysis of1the shows that EJT = andzero-phonon indicates thatlevels the excited-state Both also receive some contribution from theforvibronic 500 ±8Ocm mechanism. This process is more significant the 4T 2 exchange field is anti-ferromagnetic, isotropic and 4T state than for the 1 and may involve a slightly slightly smaller in magnitude than the ferromagnetic anharmonic low-energy vibration. Evidence for a small ground-state exchange field. Vibrationally.excited levels 4T 4T exhange contribution is found in the 1 band for both of the 2 state are examined qualitatively, aided by the systems. Factors influencing the bandshapes are analysis 4Tof the zero-phonon levels. The closely-similar examined quantitatively. The antiresonance formalism CrBr3 2 vibronic structure is also analyzed through a of Sturge er aL is applied to the anomalous lineshapes of perturbation calculation, and a good description of the certain transitions in absorption and MCD and one parsplitting, relative intensities and polarizations of the ticularly complex bandshape (that for zero-phonon levels is4Tobtained, yielding a value of UT 4T 2E—2T -÷ 2— 1 in CrCI3) is analyzed through a stat- 400cm’. Spurious 2 zero-phonon structure is found istical computation. in CrCI3 containing stacking faults, but similar complications are not found in CrBr3. Received 18 July 1977 Revised 8 June 1978 Received l8Julv 1977 Revised 8 June 1978 6. SPECTROSCOPIC STUDIES OF THE TWODIMENSIONAL MAGNETIC INSULATORS CHROMIUM TRICHLORIDE AND 7. THE SHAPE OF THE POTENTIAL ENERGY CHROMIUM TRIBROMIDE. II WELL4’OF (MIMPURITIES = Li, Na, K) IN KCI: Li~ AND Victor M. Bermudez and Donald S. McClure, KF :M Frick Chemical Laboratory, Princeton UniverJ.A. van Winsum, T. Lee, H.W. den Hartog and sity, Princeton, NJ 08540, U.S.A. A.J. Dekker, Solid State Physics Laboratory, of The splitting, spectroscopic g-factors, polarized relathe University of Groningen, I Melkweg, tive intensities and temperature dependence of the 2T 1, Groningen, The Netherlands. —
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