We present inelastic light scattering experiments on low-energy intrasubband spin-density excitations (SDE) in 12-nm-wide (001)-oriented GaAs-AlGaAs single quantum well samples with balanced Rashba and Dresselhaus spin-orbit interaction strengths. This unique symmetry causes an effective spin-orbit field either parallel or antiparallel to specific in-plane crystal directions, which supports the persistent spin helix. This results in a highly anisotropic splitting of intrasubband SDEs in the conduction band. Measurements are performed in backscattering geometry, where the SDE is formed by spin-flip intrasubband transitions. A wave-vector transfer into the two-dimensional electron system is realized by tilting the sample. By rotating the sample with a rotary stage, we can precisely map the anisotropic spin splitting, which appears for various crystal directions as a double peak line shape of the spectra. In the presence of external magnetic fields, a superposition of both, the intrinsic spin-orbit field and the external magnetic field, occurs. We analyze our experimental spectra via a lineshape analysis, based on the Lindhard-Mermin lineshape, including the effects of the anisotropic spin splitting and the external magnetic field. This allows us to quantitatively deduce the spin-orbit parameters, the electron g factor, and the single-particle relaxation time from our observations.
Philipp Nagler, Fabian Mooshammer, Mariana Ballotin, Anatolie Mitioglu, Gerd Plechinger, Sebastian Meier, Nicola Paradiso, Christoph Strunk, Rupert Huber, Alexey Chernikov, Peter C. Christianen, Christian Schüller, Tobias Korn
Two-dimensional transition-metal dichalcogenides (TMDCs) have recently emerged as a promising class of materials. A fascinating aspect of these atomically thin crystals is the possibility of combining different TMDCs into heterostructures. For several TMDC combinations, a staggered band alignment occurs, so that optically excited electron-hole pairs are spatially separated into different layers and form interlayer excitons (IEX). Here, we report on time-resolved, low-temperature photoluminescence (PL) of these IEX in a MoSe2-WSe2 heterostructure. In the time-resolved measurements, we observe indications of IEX diffusion in an inhomogeneous potential landscape. Excitation-density-dependent measurements reveal a dipolar, repulsive exciton-exciton interaction. PL measurements in applied magnetic fields show a giant valley-selective splitting of the IEX luminescence, with an effective g factor of about -15. This large value stems from the alignment of K+ and K- valleys of the constituent monolayers in our heterostructure, making intervalley transitions optically bright, so that contributions to the field-induced splitting arising from electron and hole valley magnetic moments add up. This giant splitting enables us to generate a near-unity valley polarization of interlayer excitons even under linearly polarized excitation by applying sufficiently large magnetic fields.
Malte Selig, Gunnar Berghäuser, Archana Raja, Philipp Nagler, Christian Schüller, Tony Heinz, Tobias Korn, Alexey Chernikov, Ermin Malic, Andreas Knorr
Monolayers of transition metal dichalcogenides are direct gap semiconductors, which have attracted much attention in the recent past. Due to a strong Coulomb interaction, they possess strongly bound electron-hole pairs, with binding energies of hundreds of meV which is an order of magnitude larger than in conventional materials. Here, we investigate the microscopic origin of the homogeneous linewidth and coherence lifetime of excitonic resonances in monolayer molybdenum disulfide, taking exciton phonon scattering and radiative recombination into account. We find a superlinear increasing homogeneous linewidth from 2 meV at 5K to 14 meV at room temperature corresponding to a coherence lifetime of 160 fs and 25 fs.
We demonstrate the exfoliation of large-area monolayer flakes prepared from bulk MoS2 crystals. The flakes are first characterized using Raman and photoluminescence measurements. We then utilize time-resolved Kerr rotation (TRKR) measurements to probe the valley dynamics in the monolayer flakes at low temperatures. This technique allows resonant excitation of the excitonic transitions and yields sub-picosecond resolution. We find valley lifetimes of about 40 ps at a temperature of 4 K in monolayer MoS2 for resonant excitation. With increasing temperatures, we observe a dramatic decrease of the valley lifetimes, indicating that valley dephasing is mediated by phonon-related scattering processes.
The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become
an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap
semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded twodimensional
sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. With these
techniques, few- and single-layer flakes can be prepared. Raman spectroscopy is a sensitive tool to determine the
number of layers of a flake, as two characteristic Raman modes in MoS2 shift to higher or lower frequency with
the number of layers. In addition to previously reported Raman modes in MoS2, we observe an interlayer shear
mode at very low frequencies, which also shifts with the number of layers. We use scanning Raman spectroscopy
to map and characterize MoS2 flakes.
The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become
an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap
semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded
two-dimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. Here, we
present temperature-dependent and time-resolved photoluminescence (PL) studies of single-layer MoS2 flakes.
Some of the flakes are covered with oxide layers prepared by atomic layer deposition (ALD). At low temperatures,
we clearly see two PL peaks in the as-prepared flakes without oxide layers, which we may assign to bound and
free exciton transitions. The lower-energy, bound exciton PL peak is absent in the oxide-covered flakes. In
time-resolved PL measurements, we observe very fast photocarrier recombination on the few-ps timescale at low
temperatures, with increasing photocarrier lifetimes at higher temperatures due to exciton-phonon scattering.
T. Korn, M. Griesbeck, M. Kugler, S. Furthmeier, C. Gradl, M. Hirmer, D. Schuh, W. Wegscheider, K. Korzekwa, P. Machnikowski, T. Kuhn, M. M. Glazov, E. Ya. Sherman, C. Schüller
Understanding and controlling the spin dynamics in semiconductor heterostructures is a key requirement for
the design of future spintronics devices. In GaAs-based heterostructures, electrons and holes have very different
spin dynamics. Some control over the spin-orbit fields, which drive the electron spin dynamics, is possible by
choosing the crystallographic growth axis. Here, (110)-grown structures are interesting, as the Dresselhaus spinorbit
fields are oriented along the growth axis and therefore, the typically dominant Dyakonov-Perel mechanism
is suppressed for spins oriented along this axis, leading to long spin depasing times. By contrast, hole spin
dephasing is typically very rapid due to the strong spin-orbit interaction of the p-like valence band states. For
localized holes, however, most spin dephasing mechanisms are suppressed, and long spin dephasing times may be
observed. Here, we present a study of electron and hole spin dynamics in GaAs-AlGaAs-based quantum wells.
We apply the resonant spin amplification (RSA) technique, which allows us to extract all relevant spin dynamics
parameters, such as g factors and dephasing times with high accuracy. A comparison of the measured RSA
traces with the developed theory reveals the anisotropy of the spin dephasing in the (110)-grown two-dimensional
electron systems, as well as the complex interplay between electron and hole spin and carrier dynamics in the
two-dimensional hole systems.
We report about scanning Raman experiments on, both, as deposited and nano-structured graphene flakes. The
Raman scans allow us to extract spatially resolved information about frequencies, intensities and linewidths of
the observed phonon modes. In nano-structured single-layer flakes, where periodic arrays of holes (antidots) were
fabricated by electron-beam lithography and subsequent etching, we find a systematic dependence of the phonon
frequencies, intensities and linewidths on the periods and hole sizes of the nano-patterned regions. A systematic
shift of the G mode frequency evidences a doping effect in the nano-patterned regions. In order to calibrate the
doping dependence of the G mode phonon frequency, we have investigated the position and linewidth of this
mode in a gated single-layer flake. With this calibration, we can quantitatively determine the doping level, which
is introduced via preparation of the periodic hole arrays into the samples. A comparison of G and 2D mode
frequencies allows us to identify the doping to be of p-type.
We have investigated spin-density excitations of holes in one-sided p-modulation-doped GaAs-AlGaAs single
quantum wells by means of resonant inelastic light scattering. The experiments yield a direct measure of the
Rashba spin splitting of holes in quantum wells with an asymmetric potential profile. In the low-energy range
of the inelastic light scattering spectra, we observe in all samples well-defined excitations with energies between
about 1 meV and 4 meV, which can be attributed to spin-density excitations of the two-dimensional hole
systems due to polarization selection rules. We interpret the excitations as spin-density excitations, where holes
are excited between the Rashba spin-split ground states, performing a spinflip process. Comparison to k · p
bandstructure calculations shows good agreement of the measured and calculated wave-vector-dependent spin
splittings. Details of the spectra show a distinct dependence on the directions of light polarizations with respect
to crystallographic axes. In particular, we have detected a doublet structure of the hole spin excitations, which
might be attributed to the anisotropic spin-split hole dispersion within the quantum-well plane.
Spin dynamics in zincblende two-dimensional electron systems is usually dominated by the Dyakonov-Perel
spin dephasing mechanism resulting from the underlying spin-orbit fields. An exceptional situation is realized
in symmetrically grown and doped GaAs/AlGaAs quantum wells grown along the [110] direction, where the
Rashba contribution is negligible and the effective Dresselhaus spin-orbit field is perpendicular to the sample
plane. In such a system the spin dephasing times for in- and out-of-plane crystallographic directions are expected
to be strongly different and the out-of-plane spin dephasing time is significantly enhanced as compared with
conventional systems. We observe the spin relaxation anisotropy by resonant spin amplification measurements
in a 30 nm wide double-sided symmetrically δ-doped single quantum well with a very high mobility of about
3•106 cm2/Vs at 1.5K. A comparison of the measured resonant spin amplification traces with the developed
theory taking into account the spin dephasing anisotropy yields the dephasing times whose anisotropy and magnitudes are in-line with the theoretical expectations.
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