Landau level spectroscopy of surface states in the topological insulator Bi0.91_{0.91}Sb0.09_{0.09} via magneto-optics

We have performed broad-band zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi$_{0.91}$Sb$_{0.09}$. The zero-field results allow us to measure the value of the direct band gap between the conducting $L_a$ and valence $L_s$ bands. Under applied field in the Faraday geometry (\emph{k} $||$ \emph{H} $||$ C1), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence $\omega \propto \sqrt{H}$. We discuss the ramification of this observation for the surface and bulk properties of topological insulators.Comment: 7 pages, 8 figures, March Meeting 2011 Abstract: J35.0000

Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices

We have fabricated electric double-layer field-effect devices to electrostatically dope our active materials, either $x$=0.015 Ga$_{1-x}$Mn$_x$As or $x$=3.2$\times10^{-4}$ Ga$_{1-x}$Be$_x$As. The devices are tailored for interrogation of electric field induced changes to the frequency dependent conductivity in the accumulation or depletions layers of the active material via infrared (IR) spectroscopy. The spectra of the (Ga,Be)As-based device reveal electric field induced changes to the IR conductivity consistent with an enhancement or reduction of the Drude response in the accumulation and depletion polarities, respectively. The spectroscopic features of this device are all indicative of metallic conduction within the GaAs host valence band (VB). For the (Ga,Mn)As-based device, the spectra show enhancement of the far-IR itinerant carrier response and broad mid-IR resonance upon hole accumulation, with a decrease of these features in the depletion polarity. These later spectral features demonstrate that conduction in ferromagnetic (FM) Ga$_{1-x}$Mn$_x$As is distinct from genuine metallic behavior due to extended states in the host VB. Furthermore, these data support the notion that a Mn-induced impurity band plays a vital role in the electron dynamics of FM Ga$_{1-x}$Mn$_x$As. We add, a sum-rule analysis of the spectra of our devices suggests that the Mn or Be doping does not lead to a substantial renormalization of the GaAs host VB

An infrared probe of the insulator-to-metal transition in GaMnAs and GaBeAs

We report infrared studies of the insulator-to-metal transition (IMT) in GaAs doped with either magnetic (Mn) or non-magnetic acceptors (Be). We observe a resonance with a natural assignment to impurity states in the insulating regime of Ga$_{1-x}$Mn$_x$As, which persists across the IMT to the highest doping (16%). Beyond the IMT boundary, behavior combining insulating and metallic trends also persists to the highest Mn doping. Be doped samples however, display conventional metallicity just above the critical IMT concentration, with features indicative of transport within the host valence band

Tunable hot-carrier photodetection beyond the bandgap spectral limit

The spectral response of common optoelectronic photodetectors is restricted by a cutoff wavelength limit λ that is related to the activation energy (or bandgap) of the semiconductor structure (or material) (Δ) through the relationship λ = hc/Δ. This spectral rule dominates device design and intrinsically limits the long-wavelength response of a semiconductor photodetector. Here, we report a new, long-wavelength photodetection principle based on a hot-cold hole energy transfer mechanism that overcomes this spectral limit. Hot carriers injected into a semiconductor structure interact with cold carriers and excite them to higher energy states. This enables a very long-wavelength infrared response. In our experiments, we observe a response up to 55 μm, which is tunable by varying the degree of hot-hole injection, for a GaAs/AlGaAs sample with Δ = 0.32 eV (equivalent to 3.9 μm in wavelength)

Gd 3+ And Eu 2+ Local Environment In Ca 1-xeu Xb 6 (0.0001 ≤ X ≤ 0.30) And Ca 1-xgd Xb 6 (0.0001 ≤ X ≤ 0.01)

Local environment of Gd 3+ and Eu 2+ 4f 7 ions, S =7/2, in Ca 1-xEu xB 6 (0.0001 ≤ x ≤ 0.30) and Ca xGd xB 6 (0.0001 ≤ x ≤ 0.01) is investigated by means of electron spin resonance (ESR). For x ≤ 0.001 the spectra show resolved fine structures due to the cubic crystal electric field and, in the case of Eu, the hyperfine structure due to the nuclear hyperfine field is also observed. The resonances have Lorentzian line shape, indicating insulating host for the Gd 3+ and Eu 2+ ions. As x increases, the ESR lines broaden due to local distortions caused by the Ca/Gd,Eu ions substitution. For Gd (x ≈ 0.001) and Eu (x ≈ 0.02), the spectra present superposition of Lorentzian and Dysonian resonances, suggesting a coexistence of insulating and metallic hosts for the Gd 3+ and Eu 2+ ions. The Gd 3+ and Eu 2+ fine structures are still observable up to x ≈ 0.003 for Gd and x ≈ 0.15 for Eu. For larger values of x the fine and hyperfine structures are no longer observed, the line width increases, and the line shape becomes pure Dysonian anticipating the metallic and semimetallic character of GdB 6 and EuB 6, respectively. These results clearly show that in the low concentration regime the Ca 1-xR xB 6 = Gd, Eu) systems are intrinsically inhomogeneous. No evidence of weak ferromagnetism (WF) was found in the ESR spectra of either metallic or insulating phases of these compounds, suggesting that, if WF is present in these materials, the Gd 3+ and Eu 2+ 4f 7 -electrons are shielded from the WF field. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.203715501555Young, D.P., (1999) Nature, 397, p. 412Zhitomirsky, M.E., (1999) Nature, 402, p. 251Balents, L., Varma, C.M., (2000) Phys. Rev. Lett., 84, p. 1264Barzykin, V., Gor'kov, L.P., (2000) Phys. Rev. Lett., 84, p. 2207Jarlborg, T., (2001) Physica B, 307, p. 291(2000) Phys. Rev. Lett., 85, p. 186Ceperley, D., (1999) Nature, 397, p. 386Tromp, H.J., (2000) Phys. Rev. Lett., 87, p. 16401Massidda, S., Continenza, A., De Pascale, T.M., Monnier, R., (1997) Z. Phys. B, Condens. Matter, 102, p. 83Urbano, R.R., Rettori, C., Barberis, G.E., Torelli, M., Bianchi, A., Fisk, Z., Pagliuso, P.G., Oseroff, S.B., (2002) Phys. Rev. B, 65, pp. 180407RMoriwaka, T., (2001) J. Phys. Soc. Jpn., 70, p. 341Hall, D., (2001) Phys. Rev. B, 64, p. 233105Vonlanthen, P., (2000) Phys. Rev. B, 62, p. 10076Ott, H.R., (1997) Z. Phys. B, 102, p. 337Gavilano, J.L., (2001) Phys. Rev. B, 63, pp. 140410RGiannio, K., (2002) J. Phys. Condens. Matter, 14, p. 1035Denlinger, J.D., (2002) Phys. Rev. Lett., 89, p. 157601Rhyee, J.-S., Cho, B.K., Ri, H.-C., (2003) Phys. Rev. B, 67, p. 125102Souma, S., Komatsu, H., Takahashi, T., Kaji, R., Sasaki, T., Yokoo, Y., Akimitsu, J., (2003) Phys. Rev. Lett., 90, p. 027202Terashima, T., (2000) J. Phys. Soc. Jpn., 69, p. 2423Matsubayashi, K., Maki, M., Tsuzuki, T., (2002) Nature, 420, p. 143Fisk, Z., (2002) Nature, 420, p. 144Bennett, M.C., Van Lierop, J., Berkeley, E.M., Mansfield, J.F., Henderson, C., Aronson, M.C., Young, D.P., Lacerda, A., (2004) Phys. Rev. B, 69, p. 132407Urbano, R.R., Pagliuso, P.G., Sarrao, J.L., Oseroff, S.B., Rettori, C., Bianchi, A., Nakatsuji, S., Fisk, Z., (2004) J. Magn. Magn. Mater., 272-276 (1 SUPPL.), pp. E1659Urbano, R.R., Pagliuso, P.G., Rettori, C., Schlottmann, P., Sarrao, J.L., Bianchi, A., Nakatsuji, S., Oseroff, S.B., (2005) Phys. Rev. B, 71, p. 184422Urbano, R.R., Pagliuso, P.G., Rettori, C., Oseroff, S.B., Sarrao, J.L., Schlottmann, P., Fisk, Z., (2004) Phys. Rev. B, 70, pp. 140401RAbragam, A., Bleaney, B., (1970) EPR of Transition Ions, , Clarendon Press, OxfordPake, G.E., Purcell, E.M., (1948) Phys. Rev., 74, p. 1184Bloembergen, N., (1952) J. Appl. Phys., 23, p. 1383Feher, G., Kip, A.F., (1955) Phys. Rev., 98, p. 337Dyson, F.J., (1955) Phys. Rev., 98, p. 349Essam, J.W., (1972) Phase Transitions and Critical Phenomena, 2, p. 197. , edited by C. Domb and M. S. Green (Academic Press, London)Schlottmann, P., Hellberg, C.S., (1996) J. Appl. Phys., 79, p. 6414Wigger, G.A., Beeli, C., Felder, E., Ott, H.R., Bianchi, A.D., Fisk, Z., (2004) Phys. Rev. Lett., 93, p. 14720

Gd 3+ and Eu 2+ local environment in Ca 1-xEu xB 6 (0.0001 ≤ x ≤ 0.30) and Ca 1-xGd xB 6 (0.0001 ≤ x ≤ 0.01)

Local environment of Gd 3+ and Eu 2+ 4f 7 ions, S =7/2, in Ca 1-xEu xB 6 (0.0001 ≤ x ≤ 0.30) and Ca xGd xB 6 (0.0001 ≤ x ≤ 0.01) is investigated by means of electron spin resonance (ESR). For x ≤ 0.001 the spectra show resolved fine structures due to the cubic crystal electric field and, in the case of Eu, the hyperfine structure due to the nuclear hyperfine field is also observed. The resonances have Lorentzian line shape, indicating insulating host for the Gd 3+ and Eu 2+ ions. As x increases, the ESR lines broaden due to local distortions caused by the Ca/Gd,Eu ions substitution. For Gd (x ≈ 0.001) and Eu (x ≈ 0.02), the spectra present superposition of Lorentzian and Dysonian resonances, suggesting a coexistence of insulating and metallic hosts for the Gd 3+ and Eu 2+ ions. The Gd 3+ and Eu 2+ fine structures are still observable up to x ≈ 0.003 for Gd and x ≈ 0.15 for Eu. For larger values of x the fine and hyperfine structures are no longer observed, the line width increases, and the line shape becomes pure Dysonian anticipating the metallic and semimetallic character of GdB 6 and EuB 6, respectively. These results clearly show that in the low concentration regime the Ca 1-xR xB 6 = Gd, Eu) systems are intrinsically inhomogeneous. No evidence of weak ferromagnetism (WF) was found in the ESR spectra of either metallic or insulating phases of these compounds, suggesting that, if WF is present in these materials, the Gd 3+ and Eu 2+ 4f 7 -electrons are shielded from the WF field. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA