TY  - JOUR
A1  - Wiedenmann, J.
A1  - Bocquillon, E.
A1  - Deacon, R.S.
A1  - Hartinger, S.
A1  - Herrmann, O.
A1  - Klapwijk, T.M.
A1  - Maier, L.
A1  - Ames, C.
A1  - Brüne, C.
A1  - Gould, C.
A1  - Oiwa, A.
A1  - Ishibashi, K.
A1  - Tarucha, S.
A1  - Buhmann, H.
A1  - Molenkamp, L.W.
T1  - 4π-periodic Josephson supercurrent in HgTe-based topological Josephson junctions
JF  - Nature Communications
N2  - The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator. In such a material, an unconventional induced p-wave superconductivity should occur, with a doublet of topologically protected gapless Andreev bound states, whose energies vary 4π-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction made of a HgTe weak link. The response is understood as due to a 4π-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet. Our work opens the way to more elaborate experiments to investigate the induced superconductivity in a three-dimensional insulator.
KW  - Josephson effect
KW  - supercurrent
KW  - superconductors
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-175353
VL  - 7
ER  - 
TY  - JOUR
A1  - Gulberti, A.
A1  - Moll, C.K.E.
A1  - Hamel, W.
A1  - Buhmann, C.
A1  - Koeppen, J.A.
A1  - Boelmans, K.
A1  - Zittel, S.
A1  - Gerloff, C.
A1  - Westphal, M.
A1  - Schneider, T.R.
A1  - Engel, A.K.
T1  - Predictive timing functions of cortical beta oscillations are impaired in Parkinson's disease and influenced by L-DOPA and deep brain stimulation of the subthalamic nucleus Impaired beta-band timing functions in PD
JF  - NeuroImage: Clinical
N2  - Cortex-basal ganglia circuits participate in motor timing and temporal perception, and are important for the dynamic configuration of sensorimotor networks in response to exogenous demands. In Parkinson's disease (PD) patients, rhythmic auditory stimulation (RAS) induces motor performance benefits. Hitherto, little is known concerning contributions of the basal ganglia to sensory facilitation and cortical responses to RAS in PD. Therefore, we conducted an EEG study in 12 PD patients before and after surgery for subthalamic nucleus deep brain stimulation (STN-DBS) and in 12 age-matched controls. Here we investigated the effects of levodopa and STN-DBS on resting-state EEG and on the cortical-response profile to slow and fast RAS in a passive-listening paradigm focusing on beta-band oscillations, which are important for auditory–motor coupling. The beta-modulation profile to RAS in healthy participants was characterized by local peaks preceding and following auditory stimuli. In PD patients RAS failed to induce pre-stimulus beta increases. The absence of pre-stimulus beta-band modulation may contribute to impaired rhythm perception in PD. Moreover, post-stimulus beta-band responses were highly abnormal during fast RAS in PD patients. Treatment with levodopa and STN-DBS reinstated a post-stimulus beta-modulation profile similar to controls, while STN-DBS reduced beta-band power in the resting-state. The treatment-sensitivity of beta oscillations suggests that STN-DBS may specifically improve timekeeping functions of cortical beta oscillations during fast auditory pacing.
KW  - Parkinson's disease
KW  - interval timing
KW  - beta oscillations
KW  - subthalamic nucleus
KW  - deep brain stimulation
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-150049
VL  - 9
ER  - 
TY  - JOUR
A1  - Dziom, V.
A1  - Shuvaev, A.
A1  - Pimenov, A.
A1  - Astakhov, G.V.
A1  - Ames, C.
A1  - Bendias, K.
A1  - Böttcher, J.
A1  - Tkachov, G.
A1  - Hankiewicz, E.M.
A1  - Brüne, C.
A1  - Buhmann, H.
A1  - Molenkamp, L.W.
T1  - Observation of the universal magnetoelectric effect in a 3D topological insulator
JF  - Nature Communications
N2  - The electrodynamics of topological insulators (TIs) is described by modified Maxwell’s equations, which contain additional terms that couple an electric field to a magnetization and a magnetic field to a polarization of the medium, such that the coupling coefficient is quantized in odd multiples of α/4π per surface. Here we report on the observation of this so-called topological magnetoelectric effect. We use monochromatic terahertz (THz) spectroscopy of TI structures equipped with a semitransparent gate to selectively address surface states. In high external magnetic fields, we observe a universal Faraday rotation angle equal to the fine structure constant α=e\(^{2}\)/2E\(_{0}\)hc (in SI units) when a linearly polarized THz radiation of a certain frequency passes through the two surfaces of a strained HgTe 3D TI. These experiments give insight into axion electrodynamics of TIs and may potentially be used for a metrological definition of the three basic physical constants.
KW  - topological matter
KW  - infrared spectroscopy
KW  - topological insulators
KW  - topological magnetoelectric effect
Y1  - 2017
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-170875
VL  - 8
IS  - 15197
ER  - 
TY  - JOUR
A1  - König, Markus
A1  - Baenninger, Matthias
A1  - Garcia, Andrei G. F.
A1  - Harjee, Nahid
A1  - Pruitt, Beth L.
A1  - Ames, C.
A1  - Leubner, Philipp
A1  - Brüne, Christoph
A1  - Buhmann, Hartmut
A1  - Molenkamp, Laurens W.
A1  - Goldhaber-Gordon, David
T1  - Spatially Resolved Study of Backscattering in the Quantum Spin Hall State
JF  - Physical Review X
N2  - The discovery of the quantum spin Hall (QSH) state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the quantum spin Hall state have confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples, and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using scanning gate microscopy to perturb the QSH edge states on a submicron scale, we identify well-localized scattering sites which likely limit the expected nondissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed, as expected for an ideal QSH system, and are found to be robust against weak induced potential fluctuations.
KW  - mesoscopics
KW  - topological insulators
KW  - transport
KW  - charge
KW  - wells
KW  - branched flow
KW  - nanostructures
Y1  - 2013
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-127225
SN  - 2160-3308
VL  - 3
IS  - 2
ER  - 
TY  - JOUR
A1  - Thierschmann, H.
A1  - Henke, M.
A1  - Knorr, J.
A1  - Maier, L.
A1  - Heyn, C.
A1  - Hansen, W.
A1  - Buhmann, H.
A1  - Molenkamp, L. W.
T1  - Diffusion thermopower of a serial double quantum dot
JF  - New Journal of Physics
N2  - We have experimentally studied the diffusion thermopower of a serial double quantum dot, defined electrostatically in a GaAs/AlGaAs heterostructure. We present the thermopower stability diagram for a temperature difference 1T = (20±10)mK across the device and find a maximum thermovoltage signal of several μV in the vicinity of the triple points. Along a constant energy axis in this regime, the data show a characteristic pattern which is in agreement with Mott’s relation and can be well understood within a model of sequential transport.
KW  - quantum dot
Y1  - 2013
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129714
VL  - 15
IS  - 123010
ER  - 
TY  - JOUR
A1  - Deacon, R. S.
A1  - Wiedenmann, J.
A1  - Bocquillon, E.
A1  - Domínguez, F.
A1  - Klapwijk, T. M.
A1  - Leubner, P.
A1  - Brüne, C.
A1  - Hankiewicz, E. M.
A1  - Tarucha, S.
A1  - Ishibashi, K.
A1  - Buhmann, H.
A1  - Molenkamp, L. W.
T1  - Josephson Radiation from Gapless Andreev Bound States in HgTe-Based Topological Junctions
JF  - Physical Review X
N2  - Frequency analysis of the rf emission of oscillating Josephson supercurrent is a powerful passive way of probing properties of topological Josephson junctions. In particular, measurements of the Josephson emission enable the detection of topological gapless Andreev bound states that give rise to emission at half the Josephson frequency f\(_{J}\) rather than conventional emission at f\(_{J}\). Here, we report direct measurement of rf emission spectra on Josephson junctions made of HgTe-based gate-tunable topological weak links. The emission spectra exhibit a clear signal at half the Josephson frequency f\(_{J}\)/2. The linewidths of emission lines indicate a coherence time of 0.3–4 ns for the f\(_{J}\)/2 line, much shorter than for the f\(_{J}\) line (3–4 ns). These observations strongly point towards the presence of topological gapless Andreev bound states and pave the way for a future HgTe-based platform for topological quantum computation.
KW  - condensed matter physics
KW  - Josephson junctions
KW  - topological materials
KW  - gapless Andreev bound states
Y1  - 2017
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-170969
VL  - 7
IS  - 021011
ER  - 
TY  - JOUR
A1  - Thierschmann, H
A1  - Arnold, F
A1  - Mittermüller, M
A1  - Maier, L
A1  - Heyn, C
A1  - Hansen, W
A1  - Buhmann, H
A1  - Molenkamp, L W
T1  - Thermal gating of charge currents with Coulomb coupled quantum dots
JF  - New Journal of Physics
N2  - We have observed thermal gating, i.e. electrostatic gating induced by hot electrons. The effect occurs in a device consisting of two capacitively coupled quantum dots. The double dot system is coupled to a hot electron reservoir on one side (QD1), while the conductance of the second dot (QD2) is monitored. When a bias across QD2 is applied we observe a current which is strongly dependent on the temperature of the heat reservoir. This current can be either enhanced or suppressed, depending on the relative energetic alignment of the QD levels. Thus, the system can be used to control a charge current by hot electrons.
KW  - oscillations
KW  - physics
KW  - quantum dot systems
KW  - Coulomb interaction
KW  - thermal gating
KW  - three terminal device
KW  - thermoelectrics
KW  - energy
KW  - thermopower
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-145196
VL  - 17
IS  - 113003
ER  -