Prof. Dr. Daniel Hägele

Experimental Physics
Ruhr-Universität Bochum

Contact

Hub
  • Electron spin dynamics in mesoscopic GaN nanowires
    Buß, J.H. and Fernández-Garrido, S. and Brandt, O. and Hägele, D. and Rudolph, J.
    Applied Physics Letters 114 (2019)
    The electron spin dynamics in spontaneously formed GaN nanowires (NWs) on Si(111) is investigated by time-resolved magneto-optical Kerr-rotation spectroscopy for temperatures from 15 to 260 K. A strong increase in the electron spin relaxation time by more than an order of magnitude is found as compared to bulk GaN. The temperature dependence of spin relaxation is characterized by two regimes, which are explained by a model taking into account the coexistence of two different mechanisms. As a result, the spin lifetime is limited by hyperfine interaction of localized electron spins with nuclear spins at low temperatures. The mesoscopic electron confinement in the NWs leads to a dominance of Dyakonov-Perel spin relaxation driven by interface-induced contributions at high temperatures, resulting in a slow-down, but not complete suppression of spin relaxation as compared to bulk GaN. These findings underline the important role of the high surface-to-volume ratio in NWs. © 2019 Author(s).
    view abstract10.1063/1.5080508
  • Optical excitation density dependence of spin dynamics in bulk cubic GaN
    Buß, J.H. and Schupp, T. and As, D.J. and Hägele, D. and Rudolph, J.
    Journal of Applied Physics 126 (2019)
    The influence of the optical excitation density on the electron spin dynamics is experimentally investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. The nanosecond spin relaxation times in moderately n-doped β-GaN decrease with increasing excitation density, though the effective lifetimes of the optically excited carriers are almost two orders of magnitude shorter than the spin relaxation times. This counterintuitive finding is explained by the heating of the electron system due to the excitation process. The spin relaxation times in degenerately n-doped β-GaN are found to be independent of excitation density as the very high electron Fermi temperature completely dominates over carrier heating processes in this case. © 2019 Author(s).
    view abstract10.1063/1.5123914
  • Millisecond Dynamics of the Magnetocaloric Effect in a First- and Second-Order Phase Transition Material
    Döntgen, J. and Rudolph, J. and Gottschall, T. and Gutfleisch, O. and Hägele, D.
    Energy Technology 6 (2018)
    The millisecond-dynamics of the magnetocaloric effect in Gd and La-Fe-Si-Mn, which exhibit first- and second-order phase-transitions, respectively, are investigated. Direct measurements of the adiabatic temperature change ΔT are obtained from modulation infrared thermometry with field-cycling frequencies exceeding 1 kHz at amplitudes of up to 45 mT. The peak amplitude of ΔT(T) shows a dependence on sample thickness and decreases with increasing modulation frequency for both materials despite a frequency independent susceptibility of Gd. The adiabatic ΔT depends quadratically on the external field for Gd while La−Fe−Si−Mn shows a peculiar bucket-shaped curve for temperatures below the peak maximum. A comparative study of non-caloric samples shows that dissipative heating by eddy currents or magnetic hysteresis does not explain the observed behavior. The transient ΔT(t) instead suggests a mechanism involving strong temperature gradients at the ferromagnetic–paramagnetic boundaries and underlines the importance of further dynamical studies for a fundamental understanding of the magnetocaloric effect in first-order materials. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ente.201800145
  • Modulation infrared thermometry of caloric effects at up to kHz frequencies
    Döntgen, J. and Rudolph, J. and Waske, A. and Hägele, D.
    Review of Scientific Instruments 89 (2018)
    We present a novel non-contact method for the direct measurement of caloric effects in low volume samples. The adiabatic temperature change ΔT of a magnetocaloric sample is very sensitively determined from thermal radiation. Rapid modulation of ΔT is induced by an oscillating external magnetic field. Detection of thermal radiation with a mercury-cadmium-telluride detector allows for measurements at field frequencies exceeding 1 kHz. In contrast to thermoacoustic methods, our method can be employed in vacuum which enhances adiabatic conditions especially in the case of small volume samples. Systematic measurements of the magnetocaloric effect as a function of temperature, magnetic field amplitude, and modulation frequency give a detailed picture of the thermal behavior of the sample. Highly sensitive measurements of the magnetocaloric effect are demonstrated on a 2 mm thick sample of gadolinium and a 60 μm thick Fe80B12Nb8 ribbon. © 2018 Author(s).
    view abstract10.1063/1.5008506
  • Electron spin dynamics in cubic GaN
    Buß, J.H. and Schupp, T. and As, D.J. and Brandt, O. and Hägele, D. and Rudolph, J.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    The electron spin dynamics in cubic GaN is comprehensively investigated by time-resolved magneto-optical Kerr-rotation spectroscopy over a wide range of temperatures, magnetic fields, and doping densities. The spin dynamics is found to be governed by the interplay of spin relaxation of localized electrons and Dyakonov-Perel relaxation of delocalized electrons. Localized electrons significantly contribute to spin relaxation up to room temperature at moderate doping levels, while Dyakonov-Perel relaxation dominates for high temperatures or degenerate doping levels. Quantitative agreement to Dyakonov-Perel theory requires a larger value of the spin-splitting constant than theoretically predicted. Possible reasons for this discrepancy are discussed, including the role of charged dislocations. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.235202
  • Optical manipulation of a multilevel nuclear spin in ZnO: Master equation and experiment
    Buß, J.H. and Rudolph, J. and Wassner, T.A. and Eickhoff, M. and Hägele, D.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We demonstrate the dynamics and optical control of a large quantum mechanical solid state spin system consisting of a donor electron spin strongly coupled to the 9/2 nuclear spin of In115 in the semiconductor ZnO. Comparison of electron spin dynamics observed by time-resolved pump-probe spectroscopy with density matrix theory reveals nuclear spin pumping via optically oriented electron spins, coherent spin-spin interaction, and quantization effects of the ten nuclear spin levels. Modulation of the optical electron spin orientation at frequencies above 1 MHz gives evidence for fast optical manipulation of the nuclear spin state. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.155204
  • Strain dependent electron spin dynamics in bulk cubic GaN
    Schaefer, A. and Buß, J.H. and Schupp, T. and Zado, A. and As, D.J. and Hägele, D. and Rudolph, J.
    Journal of Applied Physics 117 (2015)
    The electron spin dynamics under variable uniaxial strain is investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. Spin relaxation is found to be approximately independent of the applied strain, in complete agreement with estimates for Dyakonov-Perel spin relaxation. Our findings clearly exclude strain-induced relaxation as an effective mechanism for spin relaxation in cubic GaN. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4914069
  • Temperature dependence of the electron Landé g -factor in cubic GaN
    Buß, J.H. and Schupp, T. and As, D.J. and Hägele, D. and Rudolph, J.
    Journal of Applied Physics 118 (2015)
    The temperature dependence of the electron Landé g-factor in bulk cubic GaN is investigated over an extremely broad temperature range from 15 K up to 500 K by time-resolved Kerr-rotation spectroscopy. The g-factor is found to be approximately constant over the full investigated temperature range. Calculations by k·p-theory predict a negligible temperature dependence g(T) in complete agreement with the experiment as a consequence of the large band-gap and small spin orbit splitting in cubic GaN. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4937128
  • Temperature dependent low-field measurements of the magnetocaloric Δ T with sub-mK resolution in small volume and thin film samples
    Döntgen, J. and Rudolph, J. and Gottschall, T. and Gutfleisch, O. and Salomon, S. and Ludwig, Al. and Hägele, D.
    Applied Physics Letters 106 (2015)
    We present temperature dependent ΔT measurements of the magnetocaloric effect in a thin film sample of Gd, employing magnetomodulation and detection of thermal radiation. A bulk sample of the metamagnetic material LaFe11.05Co0.91Si1.04 shows a strong broadening of the ΔT peak for increasing field amplitudes between 4 and 45mT. Bulk Gd in comparison shows only a weak broadening. All investigated samples exhibit a clear quadratic dependence of ΔT on the external field Hext at the ΔT peak maximum, contrary to earlier predictions. An analytic expression is derived that interpolates between the Hext2-behavior at low and the well-known Hext2/3-behavior at high fields. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4906426
  • Discrete-time windows with minimal RMS bandwidth for given RMS temporal width
    Starosielec, S. and Hägele, D.
    Signal Processing 102 (2014)
    We derive a family of discrete window functions for the N-point Fourier transform for application in spectral analysis that optimize the root mean square (RMS) frequency width σω for a given temporal RMS width σ<inf>t</inf>. The window family yields as a byproduct the minimum time-bandwidth product σωσt for given σ<inf>t</inf> and N. The new windows interpolate for decreasing σ<inf>t</inf> between the popular Cosine-window and a nearly Gaussian window. The new "confined Gaussian" window function gk(cG) (with k=0,N-1) is extremely well approximated by gk(acG).© 2014 Elsevier B.V.All rights reserved.
    view abstract10.1016/j.sigpro.2014.03.033
  • Electron spin dynamics in GaN
    Rudolph, J. and Buß, J.H. and Hägele, D.
    Physica Status Solidi (B) Basic Research 251 (2014)
    Gallium nitride is a promising material system for spintronics, offering long spin relaxation times and prospects for room-temperature ferromagnetism. We review the electron spin dynamics in bulk GaN. Time-resolved magneto-optical studies of both the wurtzite and the cubic phase of GaN show the dominance of Dyakonov-Perel (DP) relaxation for free conduction band electrons. Spin relaxation in the wurtzite phase is characterized by an intrinsic spin relaxation anisotropy and the limitation of spin lifetimes by a strong Rashba term. Spin lifetimes are strongly enhanced in cubic GaN, where only a weak Dresselhaus term contributes to DP relaxation. Ion-implanted wurtzite GaN shows a strong increase of electron spin lifetimes for increasing implantation dose, caused by increasing localization of carriers. The spin dynamics of conduction band electrons in Gd-implanted GaN as a candidate for a room-temperature ferromagnetic semiconductor is also only governed by localization effects and does not show signs of an efficient exchange coupling between the electrons and the magnetic Gd ions. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201350185
  • High temperature electron spin dynamics in bulk cubic GaN: Nanosecond spin lifetimes far above room-temperature
    Buß, J.H. and Schaefer, A. and Schupp, T. and As, D.J. and Hägele, D. and Rudolph, J.
    Applied Physics Letters 105 (2014)
    The electron spin dynamics in n-doped bulk cubic GaN is investigated for very high temperatures from 293 K up to 500 K by time-resolved Kerr-rotation spectroscopy. We find extraordinarily long spin lifetimes exceeding 1 ns at 500 K. The temperature dependence of the spin relaxation time is in qualitative agreement with predictions of Dyakonov-Perel theory, while the absolute experimental times are an order of magnitude shorter than predicted. Possible reasons for this discrepancy are discussed, including the role of phase mixtures of hexagonal and cubic GaN as well as the impact of localized carriers. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4901108
  • Dyakonov-perel electron spin relaxation in a highly degenerate wurtzite semiconductor
    Rudolph, J. and Buß, J.H. and Semond, F. and Hägele, D.
    AIP Conference Proceedings 1566 (2013)
    The doping density dependence of the electron spin lifetime in n-type bulk GaN is investigated up to the highly degenerate regime by time-resolved Kerr-rotation spectroscopy. We find a non-monotonic doping density dependence with maximum spin lifetimes at the onset of degeneracy. The reduction of spin lifetimes in the degenerate regime shows a weak τs ∝ n D-2/3 density dependence, in full agreement with Dyakonov-Perel theory. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4848435
  • Electron spin relaxation dynamics in GaN: Inuence of temperature, doping density, and crystal orientation
    Buß, J.H. and Rudolph, J. and Starosielec, S. and Schaefer, A. and Semond, F. and Hägele, D.
    Proceedings of SPIE - The International Society for Optical Engineering 8623 (2013)
    We present a systematic study of electron spin relaxation in wurtzite GaN. Fast relaxation is caused by a Rashba effective magnetic field that linearly depends on the electron momentum k. The field prevents spin lifetimes to exceed 50 ps at room temperature and is the origin of an anisotropic spin relaxation tensor that we evidence by magnetic field dependent magneto-optical pump-probe measurements. In addition, the spin lifetime depends - as compared to GaAs - weaker on temperature and doping density. We give a fully analytical description of both effects based on D'yakonov-Perel' theory that describes our results quantitatively without any fitting parameter. © 2013 SPIE.
    view abstract10.1117/12.2003007
  • Long electron spin coherence in ion-implanted GaN: The role of localization
    Bu, J.H. and Rudolph, J. and Shvarkov, S. and Hardtdegen, H. and Wieck, A.D. and Hägele, D.
    Applied Physics Letters 102 (2013)
    The impact of Ga and Au ion implantation on the electron spin dynamics in bulk wurtzite GaN is studied by time-resolved Kerr-rotation spectroscopy. The spin relaxation time increases strongly by up to a factor of 20 for increasing implantation doses. This drastic increase is caused by a transition from delocalized to localized electrons. We find a characteristic change in the magnetic field dependence of spin relaxation that can be used as a sensitive probe for the degree of localization. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4804558
  • Magneto-optical studies of Gd-implanted GaN: No spin alignment of conduction band electrons
    Buß, J.H. and Rudolph, J. and Shvarkov, S. and Semond, F. and Reuter, D. and Wieck, A.D. and Hägele, D.
    Applied Physics Letters 103 (2013)
    Gd-implanted wurtzite GaN as a candidate for a ferromagnetic dilute magnetic semiconductor is investigated by time-resolved magneto-optical spectroscopy. We observe a strong increase of the electron spin lifetimes for increasing Gd doses, while the electron spin Larmor precession frequency is independent of the Gd concentration. These findings are well explained by carrier localization at defects and a negligible interaction with Gd ions. The data show that Gd-implanted GaN cannot be used for an electron spin aligner. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4819767
  • Electron spin orientation under in-plane optical excitation in GaAs quantum wells
    Pfalz, S. and Winkler, R. and Ubbelohde, N. and Hägele, D. and Oestreich, M.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    We study the optical orientation of electron spins in GaAs/AlGaAs quantum wells for excitation in the growth direction and for in-plane excitation. Time- and polarization-resolved photoluminescence excitation measurements show, for resonant excitation of the heavy-hole conduction band transition, a negligible degree of electron spin polarization for in-plane excitation and nearly 100% for excitation in the growth direction. For resonant excitation of the light-hole conduction band transition, the excited electron spin polarization has the same (opposite) direction for in-plane excitation (in the growth direction) as for excitation into the continuum. The experimental results are well explained by an accurate multiband theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.165301
  • Dyakonov-Perel electron spin relaxation in a wurtzite semiconductor: From the nondegenerate to the highly degenerate regime
    Buß, J.H. and Rudolph, J. and Starosielec, S. and Schaefer, A. and Semond, F. and Cordier, Y. and Wieck, A.D. and Hägele, D.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Electron spin lifetimes are investigated in bulk wurtzite n-GaN up to very high doping densities by time-resolved Kerr-rotation spectroscopy. The doping densities range from 5×1015 to 1.5×1019 cm-3, corresponding to Fermi temperatures as high as 1200 K. The spin lifetime shows a maximum at the onset of degeneracy. The additional determination of momentum scattering times allows for a direct comparison with an analytical expression of the spin relaxation tensor in wurtzite semiconductors for a degenerate electron gas following Dyakonov-Perel theory. We find good agreement with the experiment up to the highest densities without any fitting parameter. The only known theoretical value αe= 9.0 meV of the k-linear contribution to spin-orbit coupling in the conduction band is shown to be valid up to the highest doping densities. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.153202
  • Long room-temperature electron spin lifetimes in bulk cubic GaN
    Buß, J.H. and Rudolph, J. and Schupp, T. and As, D.J. and Lischka, K. and Hägele, D.
    Proceedings of SPIE - The International Society for Optical Engineering 7937 (2011)
    We report on very long electron spin lifetimes in cubic GaN measured by time-resolved Kerr-rotation-spectroscopy. The spin coherence times with and without external magnetic field exceed 500 ps at room temperature, despite a high n-type doping level of more than 1019 cm-3 in the bulk sample under investigation. Our findings are therefore highly relevant for spin optoelectronics in the blue wavelength regime. The spin lifetimes are found to be almost temperature independent in accord with a prediction for degenerate electron gases of Dyakonov and Perel from 1972. These results are discussed also in comparison to wurtzite GaN, which shows much shorter spin lifetimes and a dependence of spin lifetimes on the spin orientation. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstract10.1117/12.873395
  • Long room-temperature electron spin lifetimes in highly doped cubic GaN
    Buß, J.H. and Rudolph, J. and Schupp, T. and As, D.J. and Lischka, K. and Hägele, D.
    Applied Physics Letters 97 (2010)
    We report on very long electron spin relaxation times in highly n -doped bulk zincblende GaN exceeding 500 ps up to room-temperature. Time-resolved Kerr-rotation measurements show an almost temperature independent spin relaxation time between 80 and 295 K confirming an early prediction of Dyakonov and Perel for a degenerate electron gas. © 2010 American Institute of Physics.
    view abstract10.1063/1.3478838
  • Temperature dependence of electron spin relaxation in bulk GaN
    Buß, J.H. and Rudolph, J. and Natali, F. and Semond, F. and Hägele, D.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    The electron spin dynamics in n -type wurtzite GaN is studied by time-resolved Kerr rotation for temperatures from 80 to 295 K and magnetic fields up to 1 T. The temperature and magnetic field dependence of the spin-relaxation time are in good agreement with D'yakonov-Perel' theory. We present an analytic expression for the spin-relaxation tensor for semiconductors with wurtzite structure that also includes the interference of Rashba and Dresselhaus contributions. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.155216
  • Two-dimensional higher order noise spectroscopy up to radio frequencies
    Starosielec, S. and Fainblat, R. and Rudolph, J. and Hägele, D.
    Review of Scientific Instruments 81 (2010)
    Going beyond the usual determination of the frequency-resolved power spectrum of an electrical noise signal, we implement a setup for the determination of a frequency-resolved two-dimensional correlation spectrum. We demonstrate measurements of two-dimensional correlation spectra with sampling rates up to 180 MSamples/s and real-time numerical evaluation with up to 100% data coverage. As an example, the purely Gaussian behavior of 1/f resistor noise is demonstrated with unprecedented sensitivity by verifying the absence of correlations between different frequencies. Unlike the usual power spectrum, the correlation spectrum is shown to contain information on both the homogeneous and inhomogeneous linewidths of a signal, suggesting applications in spin noise spectroscopy and signal analysis in general. © 2010 American Institute of Physics.
    view abstract10.1063/1.3504369
  • magnetism

  • magnetooptics

  • semiconductors

  • spectroscopy

  • spin dynamics

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