Terahertz Radiation Driven Chiral Edge Currents in Graphene

Karch, J.; Drexler, C.; Olbrich, P.; Fehrenbacher, Markus; Hirmer, M.; Glazov, M. M.; Tarasenko, S. A.; Ivchenko, E. L.; Birkner, Bastian; Eroms, Jonathan; Weiss, Dieter; Yakimova, Rositsa; Lara‐Avila, Samuel; Kubatkin, Sergey; Ostler, Markus; Seyller, Thomas; Ganichev, Sergey

doi:10.1103/physrevlett.107.276601

Cited by 135 publications

(121 citation statements)

References 27 publications

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“…11 This is supported by weak localization measurements on the reference sample, which yield a very short intervalley scattering length of L i ≈ 40 nm and also by THz photocurrent experiments on the reference sample where photocurrents were detected in the bulk of the sample 29,30 at normal incidence, which can only be explained by a lowering of the symmetry. 31 Annealing then only changes the termination of the step edges, which influences their magnetic behavior.…”

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confidence: 52%

Annealing-induced magnetic moments detected by spin precession measurements in epitaxial graphene on SiC

et al. 2013

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We present results of nonlocal and three-terminal (3T) spin precession measurements on spin injection devices fabricated on epitaxial graphene on SiC. The measurements were performed before and after an annealing step at 150• C for 15 minutes in vacuum. The values of spin relaxation length L s and spin relaxation time τ s obtained after annealing are reduced by a factor 2 and 4, respectively, compared to those before annealing. An apparent discrepancy between spin diffusion constant D s and charge diffusion constant D c can be resolved by investigating the temperature dependence of the g factor, which is consistent with a model for paramagnetic magnetic moments. Apart from its prospects for electronic devices 1,2 singlelayer graphene (SLG) is also a very promising candidate in the field of spintronics because it is expected that spin information can be passed in graphene over long distances 3 due to the weak spin-orbit coupling and low hyperfine interaction. 4 Up to now, however, the measured spin lifetimes in exfoliated SLG (0.5 ns at RT, 5 ≈1 ns at 4 K 6 ) and also in bilayer graphene (≈2 ns at RT 7 ) on SiO 2 are still one order of magnitude smaller than in conventional semiconductor heterostructures. Even if the mobility μ for graphene on SiO 2 is modified by, e.g., ligandbound nanoparticles 8 (2700-12 000 cm 2 /V s) or by using highquality suspended graphene devices 9 (μ > 100 000 cm 2 /V s), measured spin lifetimes are below 2 ns. Similar values of τ s , slightly over 2 ns, were also reported for graphene epitaxially grown on a semi-insulating silicon carbide (SiC) substrate 10,11 using a direct nonlocal measurement 12 while a huge τ s was obtained by an indirect 13 method. In Ref. 12 fitting the Hanle curves with a g factor of 2 leads to a drastic difference between charge (D c ) and spin diffusion constant (D s ). Later the data were reinterpreted in a model employing a modified g factor.14 McCreary et al. 15 studied the influence of artificially created paramagnetic moments on spin transport in graphene, and introduced an effective exchange field model leading to an enhanced g factor. This variety of different results both at room and low temperature motivates further experiments on epitaxial graphene to understand the spin relaxation mechanism in order to control the spin information for future spintronic devices.Here we also use epitaxial graphene grown on the Si face of SiC and present nonlocal and three-terminal 16 spin precession measurements. The latter probes the spin accumulation 17 directly underneath the injector electrode induced electrically by a spin-polarized current. We compare the results before and after an annealing step and observe that our measurements after annealing can be well explained with an enhanced g factor assuming that D c and D s are equal. As the temperature dependence of this increased g factor shows a clear 1/T (paramagnetic) behavior, we believe that annealing creates local magnetic moments which influence the spin transport properties.Figure 1(a) shows a sketch of t...

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confidence: 52%

Annealing-induced magnetic moments detected by spin precession measurements in epitaxial graphene on SiC

et al. 2013

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“…The spatial beam distribution of THz radiation had an almost Gaussian profile, checked with a pyroelectric camera. [37][38][39] THz laser radiation peak intensity, I, for laser spot being of about 1.2 mm diameter on the sample, was I % 8 W/cm 2 . The profile of the microwave radiation and, in particular, the efficiency of the radiation coupling to the sample could not be determined with satisfactory accuracy.…”

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confidence: 99%

Helicity sensitive terahertz radiation detection by dual-grating-gate high electron mobility transistors

Faltermeier

Olbrich

Probst

et al. 2015

Journal of Applied Physics

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Ultrahigh sensitive sub-terahertz detection by InP-based asymmetric dual-grating-gate high-electron-mobility transistors and their broadband characteristics Appl. Phys. Lett. 104, 251114 (2014) We report on the observation of a radiation helicity sensitive photocurrent excited by terahertz (THz) radiation in dual-grating-gate (DGG) InAlAs/InGaAs/InAlAs/InP high electron mobility transistors (HEMT). For a circular polarization, the current measured between source and drain contacts changes its sign with the inversion of the radiation helicity. For elliptically polarized radiation, the total current is described by superposition of the Stokes parameters with different weights. Moreover, by variation of gate voltages applied to individual gratings, the photocurrent can be defined either by the Stokes parameter defining the radiation helicity or those for linear polarization. We show that artificial non-centrosymmetric microperiodic structures with a twodimensional electron system excited by THz radiation exhibit a dc photocurrent caused by the combined action of a spatially periodic in-plane potential and spatially modulated light. The results provide a proof of principle for the application of DGG HEMT for all-electric detection of the radiation's polarization state. V C 2015 AIP Publishing LLC. [http://dx

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“…The low symmetry results in an asymmetric scattering of carriers causing an edge photocurrent with the direction determined by the radiation helicity. This mechanism has been explored before in graphene and other 2D materials [25,26] and is also active here.…”

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Photogalvanic probing of helical edge channels in two-dimensional HgTe topological insulators

et al. 2017

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We report on the observation of a circular photogalvanic current excited by terahertz laser radiation in helical edge channels of two-dimensional (2D) HgTe topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from a right-handed to a left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a ≈3 μm distance at CNP approaches 2e 2 /h, the value characteristic for ballistic transport in 2D TIs. The data reveal that the photocurrent is caused by photoionization of helical edge electrons to the conduction band. We discuss the microscopic model of this phenomenon and compare calculations with experimental data. DOI: 10.1103/PhysRevB.95.201103 The quantum spin Hall (QSH) effect occurs in 2D TIs and rests on the existence of conducting helical edge states while the bulk is insulating [1][2][3][4]. In contrast to the quantum Hall effect, the formation of these edge states requires no magnetic field: they stem from the band inversion caused by strong spin-orbit interaction and are topologically protected by time reversal symmetry. Given that the spin-up and spin-down electrons propagate along an edge in opposite directions, i.e., the spin projection is locked to the k vector, the edge channels are helical in nature. The first experimental evidence for the QSH effect was obtained in HgTe quantum wells (QWs) [5] by observing a resistance plateau around h/2e 2 in the longitudinal resistance of a mesoscopic Hall bar. Here h is Planck's constant and e is the electron charge. This observation was further confirmed by nonlocal experiments in the ballistic [6] and diffusive [7] transport regime. Conducting edge channels were later probed by scanning SQUID microscopy [8], scanning gate microscopy [9], microwave impedance microscopy [10], and by analyzing supercurrents [11]. The spin polarization of the edge states was investigated so far by electrical means only: by detecting the spin to charge conversion in devices utilizing the inverse spin Hall effect [12] or with ferromagnetic contacts [13].Here we use circularly polarized terahertz radiation to excite selectively spin-up and spin-down electrons circling clockwise and counterclockwise around a sample. We show that the excitation causes an imbalance in the electron distribution between positive and negative wave vectors. This is probed as the associated photogalvanic [14,15] current, which reverses its direction upon switching the helicity.The experiments have been carried out on Hg 0.3 Cd 0.7 Te/HgTe/Hg 0.3 Cd 0.7 Te single QW structures with a well width of 8 nm having inverted band ordering. We used this width to maximize the energy gap to about 25 meV [4,5]. Structures were grown by molecular beam epitaxy o...

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Terahertz Radiation Driven Chiral Edge Currents in Graphene

Cited by 135 publications

References 27 publications

Annealing-induced magnetic moments detected by spin precession measurements in epitaxial graphene on SiC

Annealing-induced magnetic moments detected by spin precession measurements in epitaxial graphene on SiC

Helicity sensitive terahertz radiation detection by dual-grating-gate high electron mobility transistors

Photogalvanic probing of helical edge channels in two-dimensional HgTe topological insulators

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