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Research
Introduction
My group is currently active in the study of electronic properties of material systems. Many of the physically interesting new materials exhibit exotic emergent behavior due to highly correlated electronic interactions. Such materials promise revolutionary technological leaps in conventional and quantum computing, optical detectors and emitters, electronics, optical circuitry, energy and data storage, sensors, and metrology. Optical measurements probe materials at non-zero energy scales that correspond to interaction and gap energies. Signatures of electron interactions can manif
est in the optical response allowing their discovery and characterization. Photons are also used to create electronic excitations in the development and characterization of novel optical emitters and detectors.
The laboratory is a state-of-the-art facility with capabilities that encompass the microwave through the ultra-violet spectral regions. The workhorses of the laboratory are an optically accessible 9T magnet, far-infrared and infrared lasers, a continuous-scan Fourier-transform spectrometer, several grating spectrometers, a full array of detectors and optical components, multiple cryostats, and a laboratory wide helium recovery and liquefaction system. Over the last 15 years, we have exploited low-frequency magneto-optical measurements in the THz and near-infrared spectral regions, pioneering a polarization modulation technique that uses a heterodyne detection scheme. These tools have permitted ground-breaking work in the study of multiferroics and electro-magnons, metamaterials, topological insulators, graphene, and high-Tc cuprate superconductors.
Infrared and THz Magneto-optical measurement system
Recent seminal work has sprung from the development of a novel magneto-optical experimental technique in the IR spectral region that measures the comple
x Faraday/Kerr angle with unprecedented sensitivity.[1–3] Measurements give direct access to the full frequency dependent magneto-conductivity tensor over a spectral region corresponding to many important fundamental energy scales in condensed matter systems. In addition, Faraday measurements offer a direct probe of broken time reversal ground states of materials systems.
IR magneto-optical measurements give unique insights into strong correlation physics since interaction effects enter into σxy and σxx differently.[4–6] The magneto-optical response functions obey new sum rules[7] permitting a more global view of the effects of the interactions on the electronic structure of materials.[4,8] The technique has taken on new importance in the dawning age of intense THz light sources and new graphene-based THz detectors,[9–11] and has been incorporated into multiple magneto-optical laboratories.
High Tc cuprate superconductors
In the high Tc cuprates, our magneto-optical measurements have led to several important findings: observation of the first transport evidenc
e for small Fermi pockets in underdoped cuprates,[12] demonstration that inelastic scattering enters into the Hall conductivity differently than the longitudinal conductivity,[4,13,14] refuting of the spin - charge separation scenario for the anomalous behavior of the DC Hall Effect in cuprates, [4,13,14] observation of Fermi surface reconstruction from spin density wave gap effects in n-type cuprates, [8,15] and providing strong evidence that the famous anomalous behavior of the Hall effect over a broad range of the cuprate phase diagram is caused by inelastic scattering effects from antiferromagnetic fluctuations which is well described by Fermi-liquid transport theory that include appropriate current vertex corrections.[5,6,15]
Topological Insulators
In topological insulator systems, the bulk electronic properties were characterized[16–18] and the first spectroscopic characterization of a buried surface state was performed. The topological surface state was characterized continuously from the vicinity of the Dirac point up through the conduction band 
edge utilizing the technique (as well as cyclotron resonance) with concurrent gate modulation.[19] Layering other materials on topological insulators and characterizing an interface state is essential to the evolution of the field of topological insulators, but such interface states are inaccessible to surface probes like ARPES and STM. The interface state Dirac cone was experimentally revealed to be substantially modified[19] as theoretically predicted 25 years earlier.[20] The recently achieved improvement by two orders of magnitude in sensitivity now allows measurement of the Faraday rotation to 30 urad at THz frequencies[21] opening the door for the exploration of new physics previously inaccessible, such as the hallmark ½-quantized Hall effect predicted by topological field theory for the topological insulators in the presence of time reversal symmetry breaking effects, inherently inaccessible to dc measurements.
Multiferroics and electromagnons
Magneto-optical methods were crucial in the observation and study of electromagnons in magnetic materials. Multiferroics are materials that exhibit both magnetism and ferroelectricity. They have interesting applications, particularly in digital memory and recording industries. Studies of coupled magneto-electric excitations in multiferroics revealed that magnetic excitations that usually interact only weakly with light become electric dipole active electromagnons. This leads to a colossal magneto-d
ielectric effect in these materials. Our work proved for the first time that such electromagnons existed in YMn2O5 and TbMn2O5[25] and their excitation derive from magnon-phonon coupling.[25,26] This seminal observation has been described as crucial for positive perception of electromagnons by the scientific community (Physics 4, 88 (2011)). Studies have also been made on spin glasses – materials in which geometric frustration of the magnetic exchange interaction inhibit the formation of a magnetic state. For the first time, the low temperature phase transition in geometrically frustrated magnets ZnCr2O4 and CdCr2O4 was observed and shown to be caused by spin-phonon coupling which drives a splitting of degenerate phonon modes.[24] This work is well respected in the field of frustrated magnetism, inspiring ab-initio calculations of the observed effect as well as similar experiments on other frustrated magnets.
Graphene Plasmonics and Terahertz Photonics
The experimental discovery of two-dimensional (2D) gated graphene in 2004 by Novoselov and Geim is a seminal event in electronic materials science, ushering in a tremendous outburst of scientific activity in the study of electronic properties on this unique two-dimensional material with a gapless Dirac electronic spectrum. Terahertz radiation has many technical and scientific applications in fields ranging from security to medicine. Currently, however, THz technology is notoriously underdeveloped. Graphene plasmonics has promise of 
filling in this conspicuous gap in the electromagnetic spectrum with a robust and radically new technology. In particular highly doped graphene has recently been recognized as a powerful plasmonic material that combines many important properties at terahertz (THz) frequencies with the ability of being electrically tunable. Also, the lack of a traditional bandgap makes graphene an exceptionally versatile photonic material, and the ability to dope graphene through metallic contacts and tune the carrier density with the application of a gate offer a pathway to a variety of transformative photonic devices. Recently, we have demonstrated sensitive room temperature THz detectors that operate on a photo-thermo-electric principle with response times of 10s of femtoseconds.[9,10,27] THz absorption in a graphene element raises the temperature of the graphene carriers that then diffuse to the contacts made of dissimilar metals producing a photo-voltage proportional to the Seebeck coefficient of the graphene. We are currently developing a novel source of THz radiation based on this photo-thermo-electric effect whereby a graphene element operates as an optical mixer of two near-infrared laser beams that excite THz plasmons. [28] When coupled to an antenna, the THz charge oscillations efficiently radiate into free space.
THz studies of the quantized Hall effect
Since Faraday measurements are directly related to the Hall conductivity σxy, our measurements provide a powerful tool to study the frequency dependence of the quantum Hall effect. Despite the long history of the quantum Hall effect, the evolution of Hall plateaus as a function of frequency has not been systematically studied due to the lack of a suitable probe. Our magneto-optical system will allow these interesting measurements and can lead to a better understanding of the remarkable parts per billion precision of the quantum Hall Effect. Current work is focused on the frequency dependence of the quantized Hall conductivity in topological insulators, graphene, and GaAs heterostructures.
References
1. J. Černe, D. C. Schmadel, L. B. Rigal, and H. D. Drew, “Measurement of the infrared magneto-optic properties of thin-film metals and high temperature superconductors,” Rev. Sci. Instrum. 74, 4755–4767 (2003). http://dx.doi.org/doi:10.1063/1.1619582
2. G. S. Jenkins, D. C. Schmadel, and H. D. Drew, “Simultaneous measurement of circular dichroism and Faraday rotation at terahertz frequencies utilizing electric field sensitive detection via polarization modulation,” Rev. Sci. Instrum. 81, 083903 (2010). http://dx.doi.org/10.1063/1.3480554
3. M. Grayson, L. B. Rigal, D. C. Schmadel, H. D. Drew, and P.-J. Kung, “Spectral Measurement of the Hall Angle Response in Normal State Cuprate Superconductors,” Phys. Rev. Lett. 89, 037003 (2002). http://dx.doi.org/10.1103/PhysRevLett.89.037003
4. D. C. Schmadel, G. S. Jenkins, J. J. Tu, G. D. Gu, H. Kontani, and H. D. Drew, “Infrared Hall conductivity in optimally doped Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}: Drude behavior examined by experiment and fluctuation-exchange-model calculations,” Phys. Rev. B 75, 140506 (2007). http://dx.doi.org/10.1103/PhysRevB.75.140506
5. G. S. Jenkins, D. C. Schmadel, P. L. Bach, R. L. Greene, X. Béchamp-Laganière, G. Roberge, P. Fournier, H. Kontani, and H. D. Drew, “Origin of the anomalous Hall effect in the overdoped n-type superconductor Pr_{2−x}Ce_{x}Cuo_{4}: Current-vertex corrections due to antiferromagnetic fluctuations,” Phys. Rev. B 81, 024508 (2010). http://dx.doi.org/10.1103/PhysRevB.81.024508
6. G. S. Jenkins, D. C. Schmadel, A. B. Sushkov, G. D. Gu, H. Kontani, and H. D. Drew, “Terahertz Hall measurements on optimally doped single-crystal Bi_{2}Sr_{2}CaCu_{2}O_{8+x},” Phys. Rev. B 82, 094518 (2010). http://dx.doi.org/10.1103/PhysRevB.82.094518
7. H. D. Drew and P. Coleman, “Sum Rule for the Optical Hall Angle,” Phys. Rev. Lett. 78, 1572–1575 (1997). http://dx.doi.org/10.1103/PhysRevLett.78.1572
8. A. Zimmers, L. Shi, D. C. Schmadel, W. M. Fisher, R. L. Greene, H. D. Drew, M. Houseknecht, G. Acbas, M.-H. Kim, M.-H. Yang, J. Cerne, J. Lin, and A. Millis, “Infrared Hall effect in the electron-doped high-T_{c} cuprate Pr_{2−x}Ce_{x}CuO_{4},” Phys. Rev. B 76, 064515 (2007). http://dx.doi.org/10.1103/PhysRevB.76.064515
9. J. Yan, M.-H. Kim, J. A. Elle, A. B. Sushkov, G. S. Jenkins, H. M. Milchberg, M. S. Fuhrer, and H. D. Drew, “Dual-gated bilayer graphene hot-electron bolometer,” Nat. Nanotechnol. 7, 472–478 (2012). http://dx.doi.org/10.1038/nnano.2012.88
10. M.-H. Kim, J. Yan, R. J. Suess, T. E. Murphy, M. S. Fuhrer, and H. D. Drew, “Photothermal Response in Dual-Gated Bilayer Graphene,” Phys. Rev. Lett. 110, 247402 (2013). http://dx.doi.org/10.1103/PhysRevLett.110.247402
11. X. Cai, A. B. Sushkov, R. J. Suess, G. S. Jenkins, J. Yan, T. E. Murphy, H. D. Drew, and M. S. Fuhrer, “Sensitive Room-Temperature Terahertz Detection via Photothermoelectric Effect in Graphene,” (2013). http://arxiv.org/abs/1305.3297
12. L. B. Rigal, D. C. Schmadel, H. D. Drew, B. Maiorov, E. Osquiguil, J. S. Preston, R. Hughes, and G. D. Gu, “Magneto-optical Evidence for a Gapped Fermi Surface in Underdoped YBa_{2}Cu_{3}O_{6+x},” Phys. Rev. Lett. 93, 137002 (2004).http://dx.doi.org/10.1103/PhysRevLett.93.137002
13. J. Černe, M. Grayson, D. . Schmadel, J. Simpson, H. . Drew, R. Hughes, J. . Preston, and P.-J. Kung, “The AC Hall effect in YBCO: temperature and frequency dependence of Hall scattering,” Phys. B Condens. Matter 284–288, Part 1, 941–942 (2000). http://dx.doi.org/10.1016/S0921-4526(99)02259-0
14. J. Černe, M. Grayson, D. C. Schmadel, G. S. Jenkins, H. D. Drew, R. Hughes, A. Dabkowski, J. S. Preston, and P.-J. Kung, “Infrared Hall Effect in High- T_{c} Superconductors: Evidence for Non-Fermi-Liquid Hall Scattering,” Phys. Rev. Lett. 84, 3418–3421 (2000). http://dx.doi.org/10.1103/PhysRevLett.84.3418
15. G. S. Jenkins, D. C. Schmadel, P. L. Bach, R. L. Greene, X. Béchamp-Laganière, G. Roberge, P. Fournier, and H. D. Drew, “Terahertz magnetotransport measurements in underdoped Pr_{2−x}Ce_{x}CuO_{4} and comparison with angle-resolved photoemission,” Phys. Rev. B 79, 224525 (2009). http://dx.doi.org/10.1103/PhysRevB.79.224525
16. N. P. Butch, K. Kirshenbaum, P. Syers, A. B. Sushkov, G. S. Jenkins, H. D. Drew, and J. Paglione, “Strong surface scattering in ultrahigh-mobility Bi2 Se3 topological insulator crystals,” Phys. Rev. B 81, 241301 (2010).http://dx.doi.org/10.1103/PhysRevB.81.241301
17. A. B. Sushkov, G. S. Jenkins, D. C. Schmadel, N. P. Butch, J. Paglione, and H. D. Drew, “Far-infrared cyclotron resonance and Faraday effect in Bi2Se3,” Phys. Rev. B 82, 125110 (2010). http://dx.doi.org/10.1103/PhysRevB.82.125110
18. G. S. Jenkins, A. B. Sushkov, D. C. Schmadel, N. P. Butch, P. Syers, J. Paglione, and H. D. Drew, “Terahertz Kerr and reflectivity measurements on the topological insulator Bi2Se3,” Phys. Rev. B 82, 125120 (2010).http://dx.doi.org/10.1103/PhysRevB.82.125120
19. G. S. Jenkins, D. C. Schmadel, A. B. Sushkov, H. D. Drew, M. Bichler, G. Koblmueller, M. Brahlek, N. Bansal, and S. Oh, “Dirac cone shift of a passivated topological Bi_{2}Se_{3} interface state,” Phys. Rev. B 87, 155126 (2013). http://dx.doi.org/10.1103/PhysRevB.87.155126
20. V. Korenman and H. D. Drew, “Subbands in the gap in inverted-band semiconductor quantum wells,” Phys. Rev. B 35, 6446–6449 (1987). http://dx.doi.org/10.1103/PhysRevB.35.6446
21. G. S. Jenkins, A. B. Sushkov, D. C. Schmadel, M.-H. Kim, M. Brahlek, N. Bansal, S. Oh, and H. D. Drew, “Giant plateau in the terahertz Faraday angle in gated Bi2Se3,” Phys. Rev. B 86, 235133 (2012).http://dx.doi.org/10.1103/PhysRevB.86.235133
22. M. Dobrowolska, H. D. Drew, J. K. Furdyna, T. Ichiguchi, A. Witowski, and P. A. Wolff, “Electric-Dipole Spin Resonance of Bound Electronic States in Cd_{1-x}Mn_{x}Se,” Phys. Rev. Lett. 49, 845–848 (1982).
23. A. B. Souchkov, J. R. Simpson, M. Quijada, H. Ishibashi, N. Hur, J. S. Ahn, S. W. Cheong, A. J. Millis, and H. D. Drew, “Exchange Interaction Effects on the Optical Properties of LuMnO_{3},” Phys. Rev. Lett. 91, 027203 (2003). http://dx.doi.org/10.1103/PhysRevLett.91.02720
24. A. B. Sushkov, O. Tchernyshyov, W. R. II, S. W. Cheong, and H. D. Drew, “Probing Spin Correlations with Phonons in the Strongly Frustrated Magnet ZnCr_{2}O_{4},” Phys. Rev. Lett. 94, 137202 (2005). http://dx.doi.org/10.1103/PhysRevLett.94.137202
25. A. B. Sushkov, R. V. Aguilar, S. Park, S.-W. Cheong, and H. D. Drew, “Electromagnons in Multiferroic YMn_{2}O_{5} and TbMn_{2}O_{5},” Phys. Rev. Lett. 98, 027202 (2007). http://dx.doi.org/10.1103/PhysRevLett.98.027202
26. R. Valdés Aguilar, A. B. Sushkov, C. L. Zhang, Y. J. Choi, S.-W. Cheong, and H. D. Drew, “Colossal magnon-phonon coupling in multiferroic Eu_{075}Y_{025}MnO_{3},” Phys. Rev. B 76, 060404 (2007). http://dx.doi.org/10.1103/PhysRevB.76.060404
27. X. Cai, A. B. Sushkov, R. J. Suess, M. M. Jadidi, G. S. Jenkins, L. O. Nyakiti, R. L. Myers-Ward, J. Yan, D. K. Gaskill, T. E. Murphy, H. D. Drew, and M. S. Fuhrer, “Sensitive Room-Temperature Terahertz Detection via Photothermoelectric Effect in Graphene,” arXiv:1305.3297 [cond-mat] (2013). http://arxiv.org/abs/1305.3297
28. D. Schmadel, G. S. Jenkins, and H. D. Drew, “Proposal for a Graphene Plasmonic THz Emitter,” arXiv:1311.1605 [cond-mat] (2013). http://arxiv.org/abs/1311.1605