arXiv subjects

L. Tiemann

Publications and source records attributed to L. Tiemann.

11 recordsLinked to original sources

arXiv2020

Sublattice symmetry breaking and ultra low energy excitations in Graphene-on-hBN Heterostructures

The low-lying states of graphene contain exciting topological properties that depend on the interplay of different symmetry breaking terms. The corresponding energy gaps remained unexplored until recently, owing to the low energy scale of the terms involved (few tens of ueV). These low energy terms include sublattice splitting, the Rashba and the intrinsic spin-orbit coupling, whose balance determines the topological properties. In this work, we unravel the contributions arising from the sublattice and the intrinsic spin orbit splitting in graphene on hexagonal boron-nitride. Employing resistively-detected electron spin resonance, we measure a sublattice splitting of the order of 20E-6 eV, and confirm an intrinsic spin orbit coupling of approximately 45E-6 eV. The dominance of the latter suggests a topologically non-trivial state, involving fascinating properties. Electron spin resonance is a promising route towards unveiling the intriguing band structure at low energy scales.

cond-mat.mes-hall↗

arXiv2020

A novel planar back-gate design to control the carrier concentrations in GaAs-based double quantum wells

The precise control of a bilayer system consisting of two adjacent two-dimensional electron gases (2DEG) is demonstrated by using a novel planar back-gate approach based on ion implantation. This technique overcomes some common problems of the traditional design like the poor 2DEG mobility and leakage currents between the gate and the quantum well. Both bilayers with and without separate contacts have been prepared and tested. Tuning the electron density in one layer while keeping the second 2DEG at fixed density, one observes a dramatic increase of the carrier concentration. This tunneling resonance, which occurs at equal densities of both layers, demonstrates the separated contacts to each individual layer. In another sample with a smaller tunneling barrier and parallel contacted 2DEGs, the transition from a single 2DEG to a bilayer system is investigated at 50 mK in magnetic fields up to 12 T, showing the gate stability in high magnetic fields and very low temperatures. Transitions into an insulating (Wigner crystal) phase are observed in the individual layers in high fields at filling factors below 1/3. The absence of a fractional quantum Hall liquid at filling factor 1/5 in our structure seems to be a consequence of confining the electrons in quantum wells rather than at interfaces. The observed metal-insulator transitions appear to be nearly unaffected by the presence of the second layer separated by a barrier which is only 3 nm thick. We believe that this planar back-gate design holds great promise to produce controllable bilayers suitable to investigate the exotic (non-abelian) properties of correlated states.

cond-mat.mes-hall↗

arXiv2018

Direct observation of $Γ- X$ energy spectrum transition in narrow AlAs quantum wells

Spectra of magnetoplasma excitations have been investigated in a two-dimensional electron systems in AlAs quantum wells (QWs) of different widths. The magnetoplasma spectrum have been found to change profoundly when the quantum well width became thinner than $5.5$~nm, indicating a drastic change in the conduction electron energy spectrum. The transformation can be interpreted in terms of transition from the in-plane strongly anisotropic $X_x - X_y$ valley occupation to the out-of-plane isotropic $X_z$ valley in the QW plane. Strong enhancement of the cyclotron effective mass over the band value in narrow AlAs QWs is reported.

cond-mat.mes-hall↗

arXiv2016

Nuclear magnetic resonance and nuclear spin relaxation in AlAs quantum well probed by ESR

The study of nuclear magnetic resonance and nuclear spin-lattice relaxation was conducted in an asymmetrically doped to $n\sim1.8\times10^{11}$ cm$^{-2}$ 16 nm AlAs quantum well grown in the $[001]$-direction. Dynamic polarization of nuclear spins due to the hyperfine interaction resulted in the so-called Overhauser shift of the two-dimensional conduction electron spin resonance. The maximum shifts achieved in the experiments are several orders of magnitude smaller than in GaAs-based heterostructures indicating that hyperfine interaction is weak. The nuclear spin-lattice relaxation time extracted from the decay of Overhauser shift over time turned out to depend on the filling factor of the two-dimensional electron system. This observation indicates that nuclear spin-lattice relaxation is mostly due to the interaction between electron and nuclear spins. Overhauser shift diminishes resonantly when the RF-radiation of certain frequencies was applied to the sample. This effect served as an indirect, yet powerful method for nuclear magnetic resonance detection: NMR quadrupole splitting of $^{75}$As nuclei was clearly resolved. Theoretical calculations performed describe well these experimental findings.

cond-mat.mes-hall↗

arXiv2015

Magnetoplasma excitations of two-dimensional anisotropic heavy fermions in AlAs quantum wells

The spectra of plasma and magnetoplasma excitations in a two-dimensional system of anisotropic heavy fermions were investigated for the first time. The spectrum of microwave absorption by disk-like samples of stressed AlAs quantum wells at low electron densities showed two plasma resonances separated by a frequency gap. These two plasma resonances correspond to electron mass principle values of $(1.10 \pm 0.05) m_0$ and $(0.20 \pm 0.01) m_0$. The observed results correspond to the case of a single valley strongly anisotropic Fermi surface. It was established that electron density increase results in population of the second valley, manifesting itself as a drastic modification of the plasma spectrum. We directly determined the electron densities in each valley and the inter-valley splitting energy from the ratio of the two plasma frequencies.

cond-mat.mes-hall↗

arXiv2015

Electron spin polarization by isospin ordering in correlated two-layer quantum Hall systems

Enhancement of the electron spin polarization in a correlated two-layer two-dimensional electron system at a total Landau level filling factor of one is reported. Using resistively detected nuclear magnetic resonance, we demonstrate that the electron spin polarization of two closely-spaced two-dimensional electron systems becomes maximized when inter-layer Coulomb correlations establish spontaneous isospin ferromagnetic order. This correlation-driven polarization dominates over the spin polarizations of competing single-layer fractional Quantum Hall states under electron density imbalances.

cond-mat.str-el↗

arXiv2014

NMR profiling of quantum electron solids in high magnetic fields

When the motion of electrons is restricted to a plane under a perpendicular magnetic field B, a variety of quantum phases emerge at low temperatures whose properties are dictated by the Coulomb interaction and its interplay with disorder. At very strong B, the sequence of fractional quantum Hall (FQH) liquid phases terminates in an insulating phase, which is widely believed to be due to the solidification of electrons into domains possessing Wigner crystal (WC) order. The existence of such WC domains is signaled by the emergence of microwave pinning-mode resonances, which reflect the mechanical properties characteristic of a solid. However, the most direct manifestation of the broken translational symmetry accompanying the solidification - the spatial modulation of particles' probability amplitude - has not been observed yet. Here, we demonstrate that nuclear magnetic resonance (NMR) provides a direct probe of the density topography of electron solids in the integer and fractional quantum Hall regimes. The data uncover quantum and thermal fluctuation of lattice electrons resolved on the nanometre scale. Our results pave the way to studies of other exotic phases with non-trivial spatial spin/charge order.

cond-mat.mes-hall↗

arXiv2010

Interlayer tunneling in counterflow experiments on the excitonic condensate in quantum Hall bilayers

The effect of tunneling on the transport properties of} quantum Hall double layers in the regime of the excitonic condensate at total filling factor one is studied in counterflow experiments. If the tunnel current $I$ is smaller than a critical $I_C$, tunneling is large and is effectively shorting the two layers. For $I > I_C$ tunneling becomes negligible. Surprisingly, the transition between the two tunneling regimes has only a minor impact on the features of the filling-factor one state as observed in magneto-transport, but at currents exceeding $I_C$ the resistance along the layers increases rapidly.

cond-mat.mes-hall↗

arXiv2009

Dominant parameters for the critical tunneling current in bilayer exciton condensates

We will discuss the relevant conditions to observe a critical tunneling current [New J. Phys. 10, 045018 (2008)] in electron double-layer systems at a total filling factor of 1 and find they are related to the effective layer separation and the temperature. Our studies suggest that the intensity of the critical tunneling behavior is also directly linked to the area of the sample.

cond-mat.mes-hall↗

arXiv2008

Critical tunneling currents in the regime of bilayer excitons

We have investigated the tunneling properties of an electron double quantum well system where the lowest Landau level of each quantum well is half filled. This system is expected to be a Bose condensate of excitons. Our four-terminal dc measurements reveal a nearly vanishing interlayer voltage and the existence of critical tunneling currents which depend on the strength of the condensate state.

cond-mat.str-el↗

arXiv2007

Exciton condensate at a total filling factor of 1 in Corbino 2D electron bilayers

Magneto-transport and drag measurements on a quasi-Corbino 2D electron bilayer at the systems total filling factor 1 (v_tot=1) reveal a drag voltage that is equal in magnitude to the drive voltage as soon as the two layers begin to form the expected v_tot=1 exciton condensate. The identity of both voltages remains present even at elevated temperatures of 0.25 K. The conductance in the current carrying layer vanishes only in the limit of strong coupling between the two layers and at T->0 K which suggests the presence of an excitonic circular current.

cond-mat.mes-hall↗