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Coherent phonons in carbon nanotubes and graphene
Kim, J.H.,Nugraha, A.R.T.,Booshehri, L.G.,Haroz, E.H.,Sato, K.,Sanders, G.D.,Yee, K.J.,Lim, Y.S.,Stanton, C.J.,Saito, R.,Kono, J. Elsevier Science Publishers [etc.] 2013 Chemical physics Vol.413 No.-
We review recent studies of coherent phonons (CPs) corresponding to the radial breathing mode (RBM) and G-mode in single-wall carbon nanotubes (SWCNTs) and graphene. Because of the bandgap-diameter relationship, RBM-CPs cause bandgap oscillations in SWCNTs, modulating interband transitions at terahertz frequencies. Interband resonances enhance CP signals, allowing for chirality determination. Using pulse shaping, one can selectively excite specific-chirality SWCNTs within an ensemble. G-mode CPs exhibit temperature-dependent dephasing via interaction with RBM phonons. Our microscopic theory derives a driven oscillator equation with a density-dependent driving term, which correctly predicts CP trends within and between (2n+m) families. We also find that the diameter can initially increase or decrease. Finally, we theoretically study the radial breathing like mode in graphene nanoribbons. For excitation near the absorption edge, the driving term is much larger for zigzag nanoribbons. We also explain how the armchair nanoribbon width changes in response to laser excitation.
Bootstrapping and Askey-Wilson polynomials
Kim, J.S.,Stanton, D. Academic Press 2015 Journal of mathematical analysis and applications Vol.421 No.1
The mixed moments for the Askey-Wilson polynomials are found using a bootstrapping method and connection coefficients. A similar bootstrapping idea on generating functions gives a new Askey-Wilson generating function. Modified generating functions of orthogonal polynomials are shown to generate polynomials satisfying recurrences of known degree greater than three. An important special case of this hierarchy is a polynomial which satisfies a four term recurrence, and its combinatorics is studied.
On q-integrals over order polytopes
Kim, J.S.,Stanton, D. Academic Press 2017 Advances in Mathematics Vol. No.
A combinatorial study of multiple q-integrals is developed. This includes a q-volume of a convex polytope, which depends upon the order of q-integration. A multiple q-integral over an order polytope of a poset is interpreted as a generating function of linear extensions of the poset. Specific modifications of posets are shown to give predictable changes in q-integrals over their respective order polytopes. This method is used to combinatorially evaluate some generalized q-beta integrals. One such application is a combinatorial interpretation of a q-Selberg integral. New generating functions for generalized Gelfand-Tsetlin patterns and reverse plane partitions are established. A q-analogue to a well known result in Ehrhart theory is generalized using q-volumes and q-Ehrhart polynomials.
Moments of Askey-Wilson polynomials
Kim, J.S.,Stanton, D. Academic Press 2014 Journal of combinatorial theory. Series A Vol.125 No.-
New formulas for the nth moment μ<SUB>n</SUB>(a,b,c,d;q) of the Askey-Wilson polynomials are given. These are derived using analytic techniques, and by considering three combinatorial models for the moments: Motzkin paths, matchings, and staircase tableaux. A related positivity theorem is given and another one is conjectured.
Terahertz Emission from Nanophononic Heterostructures: the Nexus Between Scale and Origin
Hoonill Jeong,Jihoon Jeong,조영달,Eunsoon Oh,Dai-Sik Kim,Christopher J. Stanton 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.56 No.1
Newly found mechanisms of terahertz (THz) radiation via acoustic standing waves and piezoelectric fields in GaN-based strained heterostructures are discussed in terms of the relevant layer thickness. Two sample structures, with quantum well widths smaller (S1) or larger (S2) than the quasi-two-dimensional exciton Bohr radius, were investigated with increasing excitation energy. For comparative purposes, the differential reflectivity spectra (DRS) were also measured, and the signal strengths were proportional to the phase-space filling effect of quantum well carriers. The signal trace versus excitation energy in THz radiation compared to the DRS reveals that the radiation is closely related with the AlGaN/GaN interface (S1) and quantum wells (S2).
Jing Xian Quah,Stephanie Sargent,Karen Nel,Christopher M. Anstey,Tony Stanton,Kim Greaves 아시아심장혈관영상의학회 2020 Cardiovascular Imaging Asia Vol.4 No.1
Objective: Epicardial fat (EF) is metabolically active adipose tissue positioned between the epicardial surface of the heart and the pericardium. We investigated whether there is a relationship between EF and Coronary Microvascular Dysfunction (CMD) in patients presenting with chest pain and unobstructed coronary arteries. Materials and Methods: This study recruited patients referred to cardiology clinics for assessment of chest pain who subsequently underwent assessment via CT coronary angiogram (CTA). Myocardial blood flow reserve (MBFR) was assessed using myocardial contrast echocardiography. Epicardial fat volume (EFV) was measured by tracing serial slices on CTA with corresponding Hounsfield units of -195 to -45. Results: We recruited 134 participants with a mean age of 59.2 (9.8) years. CMD was present in 54 (40%) patients, and the measured mean EFV was 128 mm3 (96, 168). Fortythree patients (32%) had a coronary artery calcium score (CACS) of 0, 64 (48%) had a CACS of 1–100, 18 (13%) had a CACS of 101–400, and 9 (7%) had a CACS >400. Univariate regression analysis showed that EFV and MBFR had a correlation coefficient of R=-0.22, with a significant regression slope (β=-0.002, p=0.012). Multivariable linear regression analysis using MBFR as a continuous outcome variable revealed age (β=-0.012, p=0.011) and CACS (β=-0.003, p= 0.023) to be associated with MBFR. EFV was not associated with MBFR (β=-0.0007, p=0.538). Model repetition with MBFR as a dichotomous variable (MBFR ≥2 or <2) revealed no association with EFV. Conclusion: No relationship was found between EFV and MBFR when traditional cardiovascular risk factors and calcium score.