Interplay of Chiral and Helical States in a Quantum Spin Hall Insulator Lateral Junction.

We study the electronic transport across an electrostatically gated lateral junction in a HgTe quantum well, a canonical 2D topological insulator, with and without an applied magnetic field. We control the carrier density inside and outside a junction region independently and hence tune the number and nature of 1D edge modes propagating in each of those regions. Outside the bulk gap, the magnetic field drives the system to the quantum Hall regime, and chiral states propagate at the edge. In this regime, we observe fractional plateaus that reflect the equilibration between 1D chiral modes across the junction. As the carrier density approaches zero in the central region and at moderate fields, we observe oscillations in the resistance that we attribute to Fabry-Perot interference in the helical states, enabled by the broken time reversal symmetry. At higher fields, those oscillations disappear, in agreement with the expected absence of helical states when band inversion is lifted.

Observation of the universal magnetoelectric effect in a 3D topological insulator.

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=e2/2hc (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.

4π-periodic Josephson supercurrent in HgTe-based topological Josephson junctions.

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.

One-dimensional weak antilocalization due to the berry phase in HgTe wires.

We study the weak antilocalization (WAL) effect in the magnetoresistance of narrow HgTe wires fabricated in quantum wells with normal and inverted band ordering. Measurements at different gate voltages indicate that the WAL is only weakly affected by Rashba spin-orbit splitting and persists when the Rashba splitting is about zero. The WAL amplitude in wires with normal band ordering is an order of magnitude smaller than for wires with an inverted band structure. These observations are attributed to the Dirac-like dispersion of the energy bands in HgTe quantum wells. From the magnetic-field and temperature dependencies we extract the dephasing lengths and band Berry phases. The weaker WAL for samples with a normal band structure can be explained by a nonuniversal Berry phase which always exceeds π, the characteristic value for gapless Dirac fermions.

[Quality of fundamental movement patterns in chronic low back pain patients: a quasi-experimental cross-sectional study]. / Qualität grundlegender Bewegungsmuster bei Patienten mit chronischen lumbalen Rückenschmerzen: Eine quasi-experimentelle Querschnittsstudie.

BACKGROUND: Chronic back pain may be associated with alterations of motor control and maladaptive movement. However, instruments that systematically screen fundamental movement patterns are rare. Using a newly developed functional movement analysis, this study aims to examine whether persons with chronic unspecific low back pain display altered quality of fundamental movement patterns and whether asymmetry exists between body sides. SUBJECTS/METHODS: 20 patients with chronic back pain (♀ = 8, ♂ = 12; 49.4 ±â€Š11.3 years) and 20 healthy controls (♀ = 12, ♂ = 8; 47.7 ±â€Š10.7 years) completed the functional movement analysis. It consists of 11 items screening movements of daily life. Eight of them were to complete left and right. The overall score and the number of observed asymmetries (in items to complete left and right) constituted the primary outcomes. A preliminary analysis of reliability (four raters, four subjects) with pilot character was conducted using intraclass correlation (ICC). To compare differences in means, independent t-tests were performed. In case of significance, we calculated the effect size (Cohen's d). RESULTS: The reliability analysis showed an ICC (2.1) of 0.82 (95 % CI: 0.72 - 0.90). Patients with chronic low back pain (31.95 ±â€Š5.82) scored significantly lower than healthy subjects (44.01 ±â€Š5.27; p < 0.001, d = 2.17). Additionally, patients averaged 3.8 ±â€Š1.28 asymmetries while pain-free participants only demonstrated 1.4 ±â€Š0.94 (p < 0.001; d = 2.14). CONCLUSION: Faulty and dysbalanced movement patterns appear to be linked to chronic low back pain. Nonetheless, given an existing relation, it remains unclear whether the detected deficiencies are causes or consequences of pain. Further studies about the reliability of the presented screening tool are needed.

Giant magneto-optical faraday effect in HgTe thin films in the terahertz spectral range.

We report the observation of a giant Faraday effect, using terahertz (THz) spectroscopy on epitaxial HgTe thin films at room temperature. The effect is caused by the combination of the unique band structure and the very high electron mobility of HgTe. Our observations suggest that HgTe is a high-potential material for applications as optical isolator and modulator in the THz spectral range.

Quantum Hall effect from the topological surface states of strained bulk HgTe.

We report transport studies on a three-dimensional, 70-nm-thick HgTe layer, which is strained by epitaxial growth on a CdTe substrate. The strain induces a band gap in the otherwise semimetallic HgTe, which thus becomes a three-dimensional topological insulator. Contributions from residual bulk carriers to the transport properties of the gapped HgTe layer are negligible at mK temperatures. As a result, the sample exhibits a quantized Hall effect that results from the 2D single cone Dirac-like topological surface states.

Backscattering of Dirac fermions in HgTe quantum wells with a finite gap.

The density-dependent mobility of n-type HgTe quantum wells with inverted band ordering has been studied both experimentally and theoretically. While semiconductor heterostructures with a parabolic dispersion exhibit an increase in mobility with carrier density, high-quality HgTe quantum wells exhibit a distinct mobility maximum. We show that this mobility anomaly is due to backscattering of Dirac fermions from random fluctuations of the band gap (Dirac mass). Our findings open new avenues for the study of Dirac fermion transport with finite and random mass, which so far has been hard to access.

Direct observation of the Aharonov-Casher phase.

Ring structures fabricated from HgTe/HgCdTe quantum wells have been used to study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe nonmonotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect. This is confirmed by comparison with numerical calculations of the magnetoconductance for a multichannel ring structure within the Landauer-Büttiker formalism.

Thermopower of a Kondo spin-correlated quantum dot.

The thermopower of a Kondo-correlated gate-defined quantum dot is studied using a current heating technique. In the presence of spin correlations, the thermopower shows a clear deviation from the semiclassical Mott relation between thermopower and conductivity. The strong thermopower signal indicates a significant asymmetry in the spectral density of states of the Kondo resonance with respect to the Fermi energies of the reservoirs. The observed behavior can be explained within the framework of an Anderson-impurity model.

Probing the potential landscape inside a two-dimensional electron Gas

We report direct observations of the scattering potentials in a two-dimensional electron gas using electron-beam diffraction experiments. The diffracting objects are local density fluctuations caused by spatial and charge-state distribution of donors in the GaAs-(Al,Ga)As heterostructures. The scatterers can be manipulated externally by sample illumination or by cooling the sample down under depleted conditions.

[Differential indications for so-called "lateral release" in treatment of chondropathia patellae]. / Differenzierte Indikation zur Anwendung des sogenannten "Lateral Releases" zur Behandlung der Chondropathia patellae.

The success rate of the operation of lateral release for pain caused by the patella is reported as being between 14% and 99%. The choice between arthroscopic or open procedures does not seem to affect the results. The wide ranges of results probably reflects differences in patients selection or the method and investigations of follow up. The early term outcome usually show better results than long term follow up. This study evaluates the indications for the operation of lateral releases and discusses the result of 36 out of a total of 42 patients who were follow up for 3 years later surgery. We found that an insufficiency of dysplasia of the vastus medialis, the laxity of the capsule and soft ligaments, and a strong lateral retinaculum were important factors in the indication for this procedure. The quadriceps angle was also of prime importance, but the minor forms of patellar dysplasia played only a minor role. In the so-called hyperpression syndrome, where the patella has a strong tendency to move laterally, the simple lateral release is the single most successful operation. The indication for procedures additionally to the lateral release is examined. We found that in a case with an insufficiently guided patella, a weak capsule and ligaments, an additional capsule roughing should be performed. The presence of early degenerative changes in the joint predisposes to poor results in operations such as abrasion and pride drilling. The results in our study were assessed using the Lysholm score. Our results show that the most successful technique was the combination of an arthroscopy and an extraarticular open operation controlled by arthroscopic means. This technique was not associated with major complications such as haemarthrosis and consecutive prolonged postoperative rehabilitation. Overall we achieved a rate of 83% of good or satisfactory results at more than 3 years using the indications and techniques described above.