arXiv subjects

Matthias Baenninger

Publications and source records attributed to Matthias Baenninger.

7 recordsLinked to original sources

arXiv2015

Unexpected edge conduction in HgTe quantum wells under broken time reversal symmetry

The realization of quantum spin Hall (QSH) effect in HgTe quantum wells (QWs) is considered a milestone in the discovery of topological insulators. The QSH edge states are predicted to allow current to flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction of QSH theory that remains to be experimentally verified is the breakdown of the edge conduction under broken time reversal symmetry (TRS). Here we first establish a rigorous framework for understanding the magnetic field dependence of electrostatically gated QSH devices. We then report unexpected edge conduction under broken TRS, using a unique cryogenic microwave impedance microscopy (MIM), on a 7.5 nm HgTe QW device with an inverted band structure. At zero magnetic field and low carrier densities, clear edge conduction is observed in the local conductivity profile of this device but not in the 5.5 nm control device whose band structure is trivial. Surprisingly, the edge conduction in the 7.5 nm device persists up to 9 T with little effect from the magnetic field. This indicates physics beyond simple QSH models, possibly associated with material- specific properties, other symmetry protection and/or electron-electron interactions.

cond-mat.mes-hall↗

arXiv2012

Imaging currents in HgTe quantum wells in the quantum spin Hall regime

The quantum spin Hall (QSH) state is a genuinely new state of matter characterized by a non-trivial topology of its band structure. Its key feature is conducting edge channels whose spin polarization has potential for spintronic and quantum information applications. The QSH state was predicted and experimentally demonstrated to exist in HgTe quantum wells. The existence of the edge channels has been inferred from the fact that local and non-local conductance values in sufficiently small devices are close to the quantized values expected for ideal edge channels and from signatures of the spin polarization. The robustness of the edge channels in larger devices and the interplay between the edge channels and a conducting bulk are relatively unexplored experimentally, and are difficult to assess via transport measurements. Here we image the current in large Hallbars made from HgTe quantum wells by probing the magnetic field generated by the current using a scanning superconducting quantum interference device (SQUID). We observe that the current flows along the edge of the device in the QSH regime, and furthermore that an identifiable edge channel exists even in the presence of disorder and considerable bulk conduction as the device is gated or its temperature is raised. Our results represent a versatile method for the characterization of new quantum spin Hall materials systems, and confirm both the existence and the robustness of the predicted edge channels.

cond-mat.mes-hall↗

arXiv2012

Spatially resolved study of backscattering in the quantum spin Hall state

The discovery of the Quantum Spin Hall state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the Quantum Spin Hall state 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 sub-micron scale, we identify well-localized scattering sites which likely limit the expected non-dissipative 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.

cond-mat.mes-hall↗

arXiv2012

Colossal non-saturating linear magnetoresistance in two-dimensional electron systems at a GaAs/AlGaAs heterointerface

Engineering devices with a large electrical response to magnetic field is of fundamental importance for a range of applications such as magnetic field sensing and magnetic read-heads. We show that a colossal non-saturating linear magnetoresistance (NLMR) arises in two-dimensional electron systems hosted in a GaAs/AlGaAs heterostructure in the strongly insulating regime. When operated at high source-drain bias, the magnetoresistance of our devices increases almost linearly with magnetic field reaching nearly 10,000% at 8 Tesla, thus surpassing many known non-magnetic materials that exhibit giant NLMR. The temperature dependence and mobility analysis indicate that the NLMR has a purely classical origin, driven by nanoscale inhomogeneities. A large NLMR combined with small device dimensions makes these systems a new and attractive candidate for on-chip magnetic field sensing.

cond-mat.mes-hall↗

arXiv2010

Evidence of gate-tunable topological excitations in two-dimensional electron systems

Topological defects are ubiquitous from solid state physics to cosmology, where they drive phase transitions by proliferating as domain walls, monopoles or vortices. As quantum excitations, they often display fractional charge and anyonic statistics, making them relevant to topologically protected quantum computation, but realizing a controlled physical resource for topological excitations has been difficult. Here we report evidence of topological excitations during the localization transition in strongly interacting two-dimensional electron systems (2DESs) in GaAs/AlGaAs heterostructures. We find the electrical conductivity at low electron densities to follow a Berezinskii-Kosterlitz Thouless (BKT)-like order-disorder transition implying a gate-tunable proliferation of charged topological defects. At low temperatures, a weakening in the temperature dependence of conductivity was observed, and linked to the zero point fluctuations and delocalization of the defects. Our experiments also cast crucial insight on the nature of the ground state in strongly interacting 2DESs in presence of disorder.

cond-mat.mes-hall↗

arXiv2009

Highly enhanced thermopower in two-dimensional electron systems at milliKelvin temperatures

We report experimental observation of an unexpectedly large thermopower in mesoscopic two-dimensional (2D) electron systems on GaAs/AlGaAs heterostructures at sub-Kelvin temperatures and zero magnetic field. Unlike conventional non-magnetic high-mobility 2D systems, the thermopower in our devices increases with decreasing temperature below 0.3 K, reaching values in excess of 100 $μ$V/K, thus exceeding the free electron estimate by more than two orders of magnitude. With support from a parallel independent study of the local density of states, we suggest such a phenomenon to be linked to intrinsic localized states and many-body spin correlations in the system.

cond-mat.mes-hall↗

arXiv2007

Quantisation of Hopping Magnetoresistance Prefactor in Strongly Correlated Two-Dimensional Electron Systems

We report an universal behaviour of hopping transport in strongly interacting mesoscopic two-dimensional electron systems (2DES). In a certain window of background disorder, the resistivity at low perpendicular magnetic fields follows the expected relation $ρ(B_\perp) = ρ_{\rm{B}}\exp(αB_\perp^2)$. The prefactor $ρ_{\rm{B}}$ decreases exponentially with increasing electron density but saturates to a finite value at higher densities. Strikingly, this value is found to be universal when expressed in terms of absolute resistance and and shows quantisation at $R_{\rm{B}}\approx h/e^2$ and $R_{\rm{B}}\approx 1/2$ $ h/e^2$. We suggest a strongly correlated electronic phase as a possible explanation.

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