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Analysis of Raman modes of YNi2B2C by two-dimensional correlation spectroscopy
Journal of Molecular Structure 799 (2006) 173–176 www.elsevier.com/locate/molstruc
Analysis of Raman modes of YNi2B2C by two-dimensional correlation spectroscopy Young Mee Jung a, In-Sang Yang b
b,*
a Department of Chemistry, Kangwon National University, Chunchon 200-701, Republic of Korea Division of Nano-Sciences and Department of Physics, Ewha Womans University, Seoul 120-750, Republic of Korea
Received 30 December 2005, received in revised form 8 February 2006; accepted 6 March 2006 Available online 17 April 2006
Abstract In this study, we have applied two-dimensional (2D) correlation analysis to the temperature-dependent Raman spectra of superconducting YNi2B2C single crystal. The 2D correlation analysis on apparently single peak B1g phonon mode near 200 cm 1 shows that the peak is essentially composed of two modes at 197 and 203 cm 1, and only the weight of the two modes changes as the temperature changes. This finding is an example of the power of the 2D correlation analysis on resolving highly overlapped peaks in Raman spectra. The finding of two overlapped peaks’ nature of B1g phonon mode suggests that there are two distinctive environments of the Ni–B bonds in YNi2B2C system. Therefore, we extended 2D correlation analysis to the boron A1g mode in the range of 800–950 cm 1 in a hope to see multiple boron modes like the Ni mode. The 2D correlation analysis of the boron A1g indeed shows that the peak is composed of multiple (three) peaks at 840, 900, and 930 cm 1. Furthermore, 2D hetero-spectral correlation analysis was employed to relate the origins of the multiple phonon modes of YNi2B2C. Synchronous 2D hetero correlation spectrum obtained from the temperature-dependent Raman spectra of the Ni B1g and B A1g modes shows that a Ni B1g phonon mode at 197 cm 1 and B A1g mode at 840 cm 1 have one origin, while a Ni B1g phonon mode at 203 cm 1 and B A1g modes at 900 and 930 cm 1 arise from the other origin. 2006 Elsevier B.V. All rights reserved. Keywords: Two-dimensional correlation analysis; YNi2B2C; Borocarbides; Raman spectroscopy; Phonon modes
1. Introduction Raman spectra of YNi2B2C single crystals show Ni B1g mode near 200 cm 1 and B (boron) A1g mode in the range of 800–950 cm 1 for the case of pure 10B isotope. We have shown that the Ni B1g mode is essentially composed of two modes at 197 and 203 cm 1, and only the weight of the two modes changes as the temperature changes. The finding of double peak nature of the Ni B1g phonon mode suggested that there might be two distinctive environments of the Ni– B bonds in the YNi2B2C system. Therefore, we extend 2D correlation analysis to the B A1g mode in the range of 800–950 cm 1 in a hope to
*
Corresponding author. Tel.: +82 2 3277 2332; fax: +82 2 3277 2372. E-mail address: [email protected] (I.-S. Yang).
0022-2860/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.03.005
analyze the behaviors of the multiple boron modes as in the case of the Ni mode. The 2D correlation analysis shows that the behavior of the B A1g mode at 840 cm 1 is different from those of the other two modes at 900 and 930 cm 1, suggesting that the origin of the 840 cm 1 mode is different from that of the other two. Generalized 2D correlation spectroscopy is now well-established technique to interpret spectral data sets obtained during the observation of spectra with an external perturbation [1–4]. Some of the most notable features of 2D correlation spectra are: simplification of complex spectra consisting of many overlapped peaks; enhancement of spectral resolution by spreading peaks along the second dimension; establishment of unambiguous assignments through the correlation of the modes selectively coupled by various interaction mechanisms; and determination of the sequence of the spectral peak emergence. The details
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of this technique are described elsewhere [1–4], so no further description is given here. A very intriguing possibility in 2D correlation spectroscopy is the idea of 2D hetero-spectral correlation analysis, where two completely different types of spectra obtained for a system using multiple spectroscopic probes under a similar external perturbation are compared [4]. If there is any commonalty between the response patterns of system constituents monitored by two different probes under the same perturbation, one should be able to detect the correlation even between different classes of spectral signals. Hetero-spectral correlation has become one of the most active areas of research in 2D correlation spectroscopy.
800
Intensity (a.u.)
B1g
5.7 K 6K 10 K 13 K 17 K
1000
2. Experimental Raman spectra of YNi2B2C single crystals were measured at temperatures between 4 and 290 K using the 647 nm line of a Kr-ion laser in the back-scattering geometry. Boron in the YNi2B2C single crystals is pure 10B isotope. Raman spectra of YNi2B2C single crystals show superconducting gap-like peak near 40 cm 1 in the B2g symmetry as well as phonon modes such as the Ni B1g mode near 200 cm 1 and the B (boron) A1g mode in the range of 800–950 cm 1. 2D correlation analysis was performed using an algorithm based on the numerical method developed by Noda [1]. Prior to the 2D correlation analysis, 5-point smoothing for all the Raman spectra was carried out. A subroutine KG2D composed in Array Basic language (GRAMS/386, Galactic Inc., Salem, NH) was employed for the 2D correlation analyses [5]. 3. Results and discussion
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0 50
100
150
200
250
-1
Raman Shift (cm )
197
203
Fig. 1. Temperature-dependent Raman spectra of the Ni B1g vibrational mode of the YNi2B2C single crystal.
Fig. 1 depicts the temperature-dependent Raman spectra of the Ni B1g vibrational mode of the YNi2B2C single crystal. At temperatures lower than 15 K, there is an apparent change in the spectra below 50 cm 1 due to the superconducting gap opening in the B1g symmetry. Detailed analysis of the superconducting gap features in both B1g and B2g symmetries and their behavior in the magnetic fields was reported in our earlier papers [6,7]. In this paper, we focus on the behavior of the Ni B1g and the B A1g modes.
(a)
(b)
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Wavenumber (cm-1)
Wavenumber (cm-1)
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Wavenumber (cm-1)
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Wavenumber (cm-1)
Fig. 2. Synchronous (a) and asynchronous (b) 2D correlation spectra obtained from the temperature-dependent Raman spectra of the Ni B1g vibrational mode. Solid and dashed lines represent positive and negative cross-peaks, respectively.
Y.M. Jung, I.-S. Yang / Journal of Molecular Structure 799 (2006) 173–176
1200
1000
Intensity (a.u.)
A1g
4K 8K 12 K 16 K 30 K 60 K 100 K 160 K 220 K 290 K
800
600
400 600
700
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900
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-1
Raman Shift (cm )
Fig. 3. B (boron) A1g mode of YNi2B2C single crystals in the Raman spectra measured at various temperatures as indicated. Boron in the YNi2B2C single crystals is pure 10B isotope.
not readily noticeable in the 1D spectra of Fig. 1 are clearly observed in the synchronous 2D correlation spectrum, showing that the 2D correlation spectrum yields greater resolution than the conventional 1D spectra. The sign of the cross-peak in the asynchronous 2D correlation spectrum reveals that, with increasing temperature, the mode at 197 cm 1 gain intensity faster than the mode at 203 cm 1 lose intensity, because the mode at 197 cm 1 is more intense than that at 203 cm 1 in full population. Fig. 3 shows B A1g mode of temperature-dependent Raman spectra of YNi2B2C single crystals. Clearly, there are mainly three major peaks in the spectra. The YNi2B2C single crystals in this work were synthesized using pure isotope 10B, so that many peaks of the B A1g mode are not related with the boron isotopes. Fig. 4 shows 2D correlation spectra obtained from the whole temperature-dependent Raman spectra of B A1g mode in Fig. 3. In synchronous 2D correlation spectrum (Fig. 4(a)), three peaks at 840, 900, and 930 cm 1 are observed in the B A1g mode. Positive cross-peaks at (930, 840), (900, 840), and (930, 900) cm 1 show that intensities of three peaks decrease together with increasing temperature. From the sign of asynchronous cross-peaks (Fig. 4(b)), we can infer the following sequence of intensity changes with increasing temperature: 840 fi 900 fi 930 cm 1. Fig. 5 shows the hetero-spectral correlation spectrum of the Ni B1g and B A1g modes. The 2D hetero-spectral correlation analysis was employed to relate the origins of the multiple phonon modes of YNi2B2C. Synchronous 2D hetero correlation spectrum obtained from the temperature-
902
928
Synchronous and asynchronous 2D correlation spectra from the temperature-dependent Raman spectra of B1gvibrational mode are given in Figs. 2(a) and (b). The power spectrum extracted along the diagonal line in the synchronous spectrum is also shown at the top of Fig. 2(a), which yields two peaks at 197 and 203 cm 1. In synchronous 2D correlation spectrum, the negative crosspeak at (197, 203) cm 1 shows that the intensity of the mode at 203 cm 1 decreases, while the mode at 197 cm 1 increases. This means that with increasing temperature, the position of the mode at 203 cm 1 is shifted to 197 cm 1. These two modes at 197 and 203 cm 1 that are
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(b)
840
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Wavenumber (cm-1)
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Fig. 4. Synchronous (a) and asynchronous (b) 2D correlation spectra obtained from the temperature-dependent B A1g Raman spectra. Solid and dashed lines represent positive and negative cross-peaks, respectively.
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only the weight of the two modes changes as the temperature changes. The behavior of the B A1g mode at 840 cm 1 is different from those of the other two A1g modes at 900 and 930 cm 1, suggesting that the origin of the 840 cm 1 mode is different from that of the other two. Furthermore, 2D hetero-spectral correlation analysis was employed to relate the origins of the multiple phonon modes of YNi2B2C. Synchronous 2D hetero correlation spectrum obtained from the temperature-dependent Raman spectra of the Ni B1g and B A1g modes shows that a Ni B1g phonon mode at 197 cm 1 and B A1g mode at 840 cm 1 have one origin, while a Ni B1g phonon mode at 203 cm 1 and B A1g modes at 900 and 930 cm 1 arise from the other origin.
A1g
Wavenumber (cm-1)
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900 880 860 840
820 800 180
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B 1g Wavenumber (cm ) -1
Fig. 5. Synchronous 2D hetero-spectral correlation spectrum obtained from temperature-dependent Raman spectra of the Ni B1g and the B A1g vibrational modes. Solid and dashed lines represent positive and negative cross-peaks, respectively.
dependent Raman spectra of the Ni B1g and B A1g modes shows that a Ni B1g phonon mode at 197 cm 1 and B A1g mode at 840 cm 1 have one origin, while a Ni B1g phonon mode at 203 cm 1 and B A1g modes at 900 and 930 cm 1 arise from the other origin. 4. Conclusion We have applied 2D correlation analysis to the temperature-dependent Ni (nickel) B1g and B (boron) A1g phonon modes of Raman spectra of YNi2B2C single crystals. The 2D correlation analysis on apparently single peak Ni B1g phonon mode near 200 cm 1 shows that the peak is essentially composed of two modes at 197 and 203 cm 1, and
Acknowledgment I.-S.Y. acknowledges the financial support from the 2003 Advanced Basic Research Laboratory (ABRL) program R14-2003-027-01001-0. References [1] I. Noda, Appl. Spectrosc. 47 (1993) 47. [2] I. Noda, A.E. Dowrey, C. Marcott, G.M. Story, Y. Ozaki, Appl. Spectrosc. 54 (2000) 236A. [3] I. Noda, in: J.M. Chalmers, P.R. Griffiths (Eds.), Handbook of Vibrational Spectroscopy, vol. 3, Wiley, New York, 2002, pp. 2123– 2134. [4] I. Noda, Y. Ozaki, in: Two-Dimensional Correlation Spectroscopy: Applications in Vibrational Spectroscopy, Wiley, New York, 2004. [5] The program is available from the home page of Professor Yukihiro Ozaki (http://science.kwansei.ac.jp/~ozaki/) of Kwansei Gakuin University, Sanda, Japan. [6] Y.M. Jung, S.B. Kim, I.-S. Yang, J. Korean Phys. Soc. 45 (2004) 652. [7] I.-S. Yang, Y.M. Jung, S.B. Kim, M.V. Klein, Int. J. Mod. Phys. B 19 (2005) 281.
Analysis of Raman modes of YNi2B2C by two-dimensional correlation spectroscopy Young Mee Jung a, In-Sang Yang b
b,*
a Department of Chemistry, Kangwon National University, Chunchon 200-701, Republic of Korea Division of Nano-Sciences and Department of Physics, Ewha Womans University, Seoul 120-750, Republic of Korea
Received 30 December 2005, received in revised form 8 February 2006; accepted 6 March 2006 Available online 17 April 2006
Abstract In this study, we have applied two-dimensional (2D) correlation analysis to the temperature-dependent Raman spectra of superconducting YNi2B2C single crystal. The 2D correlation analysis on apparently single peak B1g phonon mode near 200 cm 1 shows that the peak is essentially composed of two modes at 197 and 203 cm 1, and only the weight of the two modes changes as the temperature changes. This finding is an example of the power of the 2D correlation analysis on resolving highly overlapped peaks in Raman spectra. The finding of two overlapped peaks’ nature of B1g phonon mode suggests that there are two distinctive environments of the Ni–B bonds in YNi2B2C system. Therefore, we extended 2D correlation analysis to the boron A1g mode in the range of 800–950 cm 1 in a hope to see multiple boron modes like the Ni mode. The 2D correlation analysis of the boron A1g indeed shows that the peak is composed of multiple (three) peaks at 840, 900, and 930 cm 1. Furthermore, 2D hetero-spectral correlation analysis was employed to relate the origins of the multiple phonon modes of YNi2B2C. Synchronous 2D hetero correlation spectrum obtained from the temperature-dependent Raman spectra of the Ni B1g and B A1g modes shows that a Ni B1g phonon mode at 197 cm 1 and B A1g mode at 840 cm 1 have one origin, while a Ni B1g phonon mode at 203 cm 1 and B A1g modes at 900 and 930 cm 1 arise from the other origin. 2006 Elsevier B.V. All rights reserved. Keywords: Two-dimensional correlation analysis; YNi2B2C; Borocarbides; Raman spectroscopy; Phonon modes
1. Introduction Raman spectra of YNi2B2C single crystals show Ni B1g mode near 200 cm 1 and B (boron) A1g mode in the range of 800–950 cm 1 for the case of pure 10B isotope. We have shown that the Ni B1g mode is essentially composed of two modes at 197 and 203 cm 1, and only the weight of the two modes changes as the temperature changes. The finding of double peak nature of the Ni B1g phonon mode suggested that there might be two distinctive environments of the Ni– B bonds in the YNi2B2C system. Therefore, we extend 2D correlation analysis to the B A1g mode in the range of 800–950 cm 1 in a hope to
*
Corresponding author. Tel.: +82 2 3277 2332; fax: +82 2 3277 2372. E-mail address: [email protected] (I.-S. Yang).
0022-2860/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.03.005
analyze the behaviors of the multiple boron modes as in the case of the Ni mode. The 2D correlation analysis shows that the behavior of the B A1g mode at 840 cm 1 is different from those of the other two modes at 900 and 930 cm 1, suggesting that the origin of the 840 cm 1 mode is different from that of the other two. Generalized 2D correlation spectroscopy is now well-established technique to interpret spectral data sets obtained during the observation of spectra with an external perturbation [1–4]. Some of the most notable features of 2D correlation spectra are: simplification of complex spectra consisting of many overlapped peaks; enhancement of spectral resolution by spreading peaks along the second dimension; establishment of unambiguous assignments through the correlation of the modes selectively coupled by various interaction mechanisms; and determination of the sequence of the spectral peak emergence. The details
174
Y.M. Jung, I.-S. Yang / Journal of Molecular Structure 799 (2006) 173–176
of this technique are described elsewhere [1–4], so no further description is given here. A very intriguing possibility in 2D correlation spectroscopy is the idea of 2D hetero-spectral correlation analysis, where two completely different types of spectra obtained for a system using multiple spectroscopic probes under a similar external perturbation are compared [4]. If there is any commonalty between the response patterns of system constituents monitored by two different probes under the same perturbation, one should be able to detect the correlation even between different classes of spectral signals. Hetero-spectral correlation has become one of the most active areas of research in 2D correlation spectroscopy.
800
Intensity (a.u.)
B1g
5.7 K 6K 10 K 13 K 17 K
1000
2. Experimental Raman spectra of YNi2B2C single crystals were measured at temperatures between 4 and 290 K using the 647 nm line of a Kr-ion laser in the back-scattering geometry. Boron in the YNi2B2C single crystals is pure 10B isotope. Raman spectra of YNi2B2C single crystals show superconducting gap-like peak near 40 cm 1 in the B2g symmetry as well as phonon modes such as the Ni B1g mode near 200 cm 1 and the B (boron) A1g mode in the range of 800–950 cm 1. 2D correlation analysis was performed using an algorithm based on the numerical method developed by Noda [1]. Prior to the 2D correlation analysis, 5-point smoothing for all the Raman spectra was carried out. A subroutine KG2D composed in Array Basic language (GRAMS/386, Galactic Inc., Salem, NH) was employed for the 2D correlation analyses [5]. 3. Results and discussion
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400
200
0 50
100
150
200
250
-1
Raman Shift (cm )
197
203
Fig. 1. Temperature-dependent Raman spectra of the Ni B1g vibrational mode of the YNi2B2C single crystal.
Fig. 1 depicts the temperature-dependent Raman spectra of the Ni B1g vibrational mode of the YNi2B2C single crystal. At temperatures lower than 15 K, there is an apparent change in the spectra below 50 cm 1 due to the superconducting gap opening in the B1g symmetry. Detailed analysis of the superconducting gap features in both B1g and B2g symmetries and their behavior in the magnetic fields was reported in our earlier papers [6,7]. In this paper, we focus on the behavior of the Ni B1g and the B A1g modes.
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(b)
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Fig. 2. Synchronous (a) and asynchronous (b) 2D correlation spectra obtained from the temperature-dependent Raman spectra of the Ni B1g vibrational mode. Solid and dashed lines represent positive and negative cross-peaks, respectively.
Y.M. Jung, I.-S. Yang / Journal of Molecular Structure 799 (2006) 173–176
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Intensity (a.u.)
A1g
4K 8K 12 K 16 K 30 K 60 K 100 K 160 K 220 K 290 K
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Raman Shift (cm )
Fig. 3. B (boron) A1g mode of YNi2B2C single crystals in the Raman spectra measured at various temperatures as indicated. Boron in the YNi2B2C single crystals is pure 10B isotope.
not readily noticeable in the 1D spectra of Fig. 1 are clearly observed in the synchronous 2D correlation spectrum, showing that the 2D correlation spectrum yields greater resolution than the conventional 1D spectra. The sign of the cross-peak in the asynchronous 2D correlation spectrum reveals that, with increasing temperature, the mode at 197 cm 1 gain intensity faster than the mode at 203 cm 1 lose intensity, because the mode at 197 cm 1 is more intense than that at 203 cm 1 in full population. Fig. 3 shows B A1g mode of temperature-dependent Raman spectra of YNi2B2C single crystals. Clearly, there are mainly three major peaks in the spectra. The YNi2B2C single crystals in this work were synthesized using pure isotope 10B, so that many peaks of the B A1g mode are not related with the boron isotopes. Fig. 4 shows 2D correlation spectra obtained from the whole temperature-dependent Raman spectra of B A1g mode in Fig. 3. In synchronous 2D correlation spectrum (Fig. 4(a)), three peaks at 840, 900, and 930 cm 1 are observed in the B A1g mode. Positive cross-peaks at (930, 840), (900, 840), and (930, 900) cm 1 show that intensities of three peaks decrease together with increasing temperature. From the sign of asynchronous cross-peaks (Fig. 4(b)), we can infer the following sequence of intensity changes with increasing temperature: 840 fi 900 fi 930 cm 1. Fig. 5 shows the hetero-spectral correlation spectrum of the Ni B1g and B A1g modes. The 2D hetero-spectral correlation analysis was employed to relate the origins of the multiple phonon modes of YNi2B2C. Synchronous 2D hetero correlation spectrum obtained from the temperature-
902
928
Synchronous and asynchronous 2D correlation spectra from the temperature-dependent Raman spectra of B1gvibrational mode are given in Figs. 2(a) and (b). The power spectrum extracted along the diagonal line in the synchronous spectrum is also shown at the top of Fig. 2(a), which yields two peaks at 197 and 203 cm 1. In synchronous 2D correlation spectrum, the negative crosspeak at (197, 203) cm 1 shows that the intensity of the mode at 203 cm 1 decreases, while the mode at 197 cm 1 increases. This means that with increasing temperature, the position of the mode at 203 cm 1 is shifted to 197 cm 1. These two modes at 197 and 203 cm 1 that are
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Wavenumber (cm-1)
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Fig. 4. Synchronous (a) and asynchronous (b) 2D correlation spectra obtained from the temperature-dependent B A1g Raman spectra. Solid and dashed lines represent positive and negative cross-peaks, respectively.
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only the weight of the two modes changes as the temperature changes. The behavior of the B A1g mode at 840 cm 1 is different from those of the other two A1g modes at 900 and 930 cm 1, suggesting that the origin of the 840 cm 1 mode is different from that of the other two. Furthermore, 2D hetero-spectral correlation analysis was employed to relate the origins of the multiple phonon modes of YNi2B2C. Synchronous 2D hetero correlation spectrum obtained from the temperature-dependent Raman spectra of the Ni B1g and B A1g modes shows that a Ni B1g phonon mode at 197 cm 1 and B A1g mode at 840 cm 1 have one origin, while a Ni B1g phonon mode at 203 cm 1 and B A1g modes at 900 and 930 cm 1 arise from the other origin.
A1g
Wavenumber (cm-1)
940 920
900 880 860 840
820 800 180
190
200
210
220
B 1g Wavenumber (cm ) -1
Fig. 5. Synchronous 2D hetero-spectral correlation spectrum obtained from temperature-dependent Raman spectra of the Ni B1g and the B A1g vibrational modes. Solid and dashed lines represent positive and negative cross-peaks, respectively.
dependent Raman spectra of the Ni B1g and B A1g modes shows that a Ni B1g phonon mode at 197 cm 1 and B A1g mode at 840 cm 1 have one origin, while a Ni B1g phonon mode at 203 cm 1 and B A1g modes at 900 and 930 cm 1 arise from the other origin. 4. Conclusion We have applied 2D correlation analysis to the temperature-dependent Ni (nickel) B1g and B (boron) A1g phonon modes of Raman spectra of YNi2B2C single crystals. The 2D correlation analysis on apparently single peak Ni B1g phonon mode near 200 cm 1 shows that the peak is essentially composed of two modes at 197 and 203 cm 1, and
Acknowledgment I.-S.Y. acknowledges the financial support from the 2003 Advanced Basic Research Laboratory (ABRL) program R14-2003-027-01001-0. References [1] I. Noda, Appl. Spectrosc. 47 (1993) 47. [2] I. Noda, A.E. Dowrey, C. Marcott, G.M. Story, Y. Ozaki, Appl. Spectrosc. 54 (2000) 236A. [3] I. Noda, in: J.M. Chalmers, P.R. Griffiths (Eds.), Handbook of Vibrational Spectroscopy, vol. 3, Wiley, New York, 2002, pp. 2123– 2134. [4] I. Noda, Y. Ozaki, in: Two-Dimensional Correlation Spectroscopy: Applications in Vibrational Spectroscopy, Wiley, New York, 2004. [5] The program is available from the home page of Professor Yukihiro Ozaki (http://science.kwansei.ac.jp/~ozaki/) of Kwansei Gakuin University, Sanda, Japan. [6] Y.M. Jung, S.B. Kim, I.-S. Yang, J. Korean Phys. Soc. 45 (2004) 652. [7] I.-S. Yang, Y.M. Jung, S.B. Kim, M.V. Klein, Int. J. Mod. Phys. B 19 (2005) 281.