Ph y s ic s & Na n o s t r u ct u r e Ph y s ic s page 128 of Ruthenate and and Ruthenate Strontium of This eutectic system [5] is grown by adding ruthenium Abstract: Contact: [email protected], [email protected],[email protected] NNIN REUMentor(s): Yiqun A. Ying, NealStaley, ConorPuls;Physics,ThePennsylvaniaStateUniversity NNIN REUPrincipalInvestigator(s):Dr. Ying Liu,DepartmentofPhysics,ThePennsylvaniaStateUniversity NNIN REUSite:PennStateNanofabricationLaboratory, PennsylvaniaStateUniversity, StateCollege,PA Engineering, UniversityofCalifornia,Berkeley Systems Eutectic Eric Hao of Studies Topography Raman atc (eo a eti temperature certain a (below lattice If these Cooper Pairs can survive the vibrations of the crystal special electron-phonon interactions that create Cooper Pairs. simply,on it relies put to that which, says the is superconductivity Cooper,Bardeen, BCS after (named Theory Schrieffer),and explaining for theory main The superconductor [1-4] that has that [1-4] superconductor we are interested in composes of Sr of composes in interested are we system superconductor The structures. crystal complicated characteristics (highest 140K); however temperature these critical have high extremely its for studied and well-known is range of 0-10K. Another type of superconductor— the in temperatures critical have superconductor— s-wave—which typically superconductors of These superconductors. all of majority type a constitutes simplest explains only the however, theory, This superconducting. is Types ofSuperconductors: metal —a conventional —a metal as structure similar exhibits but

odcig rniin eprtr (T temperature transition conducting near the interface, suggesting the nearsuggesting interface, the hardened phonons the noticed we interface, the across scans line performed we As analyzed. and structure—and the emitted photon was recaptured bonding specific on dependent phonons—energy scatter electrons excited where shone sample, the was onto laser of A stiffness. method phonon measuring convenient a states spectroscopy, electronic Raman the employed we this, with examine strongly. To modified materials, two the between interface sharp atomically the at occurs uecnutr (high superconductors similar structure () compared to p-wave superconductor is significant because of its Sr We report unexpected phenomena observed on the totu rteae (Sr ruthenate strontium in embedded islands (Ru) phase correlates withphonons. twice that of pure Sr 2 RuO 4 -Ru eutectic system featuring ruthenium featuring system eutectic -Ru -wave superconductor (T superconductor s-wave 2 RuO T c in range of range in T 4 2 c . This enhancement of a -wave superconductor p-wave wv—n ruthenium d-wave—and RuO uecnutr) It superconductors). 2 RuO T c , hn h material the then ), 4 wt a super- a with ) 4 —a chiral —a T c of the eutectic the of -wave (1.5K), s-wave c c d-wave— nearly ) = 0.5K). = d-wave p-wave s-wave of power, was shone onto a sample of the eutectic phase of phase eutectic the of sample milliWattsa onto shone 13 was power, of at operating laser, wavelength nm 514.5 A by measuringitsresultingshift inenergy. photon containing the “molecular imprint.” This is quantified a on shooting photon at by the sample, and works collecting based the inelastically it reflected summarize, To materials structure. bonding discriminates spectroscopy Raman Procedure: phonon detection. of use through restructuring crystal of evidence find to aims superconductivity.its destroy which project impurities, This interesting because Sr an enhanced region of superconductivity: of region enhanced an exhibit materials two these between interface The length. in ruthenium forming islands about 1-2 µm in width and as long as 10 µm excess with substrate, ruthenate strontium a to ruthenium (brightlylitareas) andSr The 2009 NNIN REU Research Accomplishments Research REU NNIN 2009 The Figure 1:Eutecticsystemconsistingof 2 RuO 4 usually is extremely sensitive to T c = 3K. This is This 3K. = 2 RuO 4 . and oxygen reflected a change in the phonons associated with the crystal structure. This change in crystal structure was correlated with the change in the critical temperature of the 3K enhanced phase, suggesting that phonons play an important role in the enhancement of the eutectic system. Further suggested experiments include temperature- dependent as well as polarization dependent Raman spectroscopy measurements in order to further quantify this “phonon stiffening.”

Acknowledgments: The author would like to thank Dr. Y. Liu; Y. A. Ying; N. Staley; C. Puls; J. Stitt; Dr. J. Robinson; Dr. R. Redwing, as well as the National Science Foundation; The NNIN REU Figure 2: Spectrum of bulk ruthenium (bottom curve) Program; the Department of Energy; and the Department of Defense, Army Research Office for funding. and bulk Sr2RuO4 (top curve). P

References: s y h Sr2RuO4-Ru. Its spot size was determined using an optics

formula, which came out to a 349 nm lateral resolution. First, [1] A. P. Mackenzie, Y. Maeno, Rev. Mod. Phys. 75, 657 (2003). C I

[2] K.D. Nelson, Z.Q. Mao, Y. Maeno, Y. Liu, Science 306, 1151 (2004). s we measured the Raman spectrum at bulk Sr2RuO4, which & N exhibited apex oxygen vibrations at 537.5 cm-1 and strontium [3] J. Xia, Y. Maeno, P. T. Beyersdorf, M. M. Fejer, A. Kapitulnik, Phys. -1 Rev. Lett. 97, 167002 (2006).

vibrations at 203.11 cm . We then measured the Raman s o n a [4] F. Kidwingira, J. D. Strand, D. J. van Harlingen, Y. Maeno, Science spectrum at bulk ruthenium, which exhibited vibrations at 314, 1267 (2006). -1

190 cm . Since the enhanced phase occurs outside of the [5] Y. Maeno et al., Phys. Rev. Lett. 81, 3765 (1998). t ruthenium island, we chose to measure the vibrational shift u r T C of Sr2RuO4. e r u We first performed line scans, starting at bulk P

Sr2RuO4 and moving in 349 nm increments, s y h collecting spectra at each point, until the strontium and oxygen vibrational modes C I s disappeared inside the ruthenium island.

Results: We received data ranging from 50 cm-1 to 1300 cm-1. First, we cropped out the irrelevant data, obtaining two sets (one for strontium and one for oxygen). We removed the linear bias associated with each region, and fit Gaussian peaks, recording its central peak position. We noticed a shift of about 8.3 cm-1 in the oxygen vibration, a change of about 1.3%, as well as a shift of about 1.6 cm-1 in the strontium vibration, corresponding to a change of about 0.8%. We were also successful at reproducing this phenomenon, through repeated line scans in different areas, and different islands.

Conclusion: Figure 3: Sr vibration line scans with peak positions and distance from island. Raman spectroscopy data gave evidence of so- called “phonon stiffening.” The change in the Figure 4: Oxygen vibration line scans with vibrational frequencies of both the strontium peak positions and distance from island.

The 2009 NNIN REU Research Accomplishments page 129