arXiv subjects

M. Hilke

Publications and source records attributed to M. Hilke.

At least 19 recordsLinked to original sources

arXiv2025

Resonant Magneto-phonon Emission by Supersonic Electrons in Ultra-high Mobility Two-dimensional System

We investigate resonant acoustic phonon scattering in the magneto-resistivity of an ultra-high mobility two-dimensional electron gas system subject to DC current in the temperature range 10 mK to 3.9 K. For a DC current density of $\sim$1.1 A/m, the induced carrier drift velocity $v_{drift}$ becomes equal to the speed of sound $s \sim$ 3 km/s. When $v_{drift} \gtrsim s$ very strong resonant features with only weak temperature dependence are observed and identified as phonon-induced resistance oscillations at and above the "sound barrier". Their behavior contrasts with that in the subsonic regime ($v_{drift} < s$) where resonant acoustic phonon scattering is strongly suppressed when the temperature is reduced unless amplified with quasi-elastic inter-Landau-level scattering. Our observations are compared to recent theoretical predictions from which we can extract a dimensionless electron-phonon coupling constant of $g^{2}$=0.0016 for the strong non-linear transport regime. We find evidence for a predicted oscillation phase change ' effect on traversing the "sound barrier". Crossing the "sound barrier" fundamentally alters the resulting phonon emission processes, and the applied magnetic field results in pronounced and sharp resonant phonon emission due to Landau level quantization.

cond-mat.mes-hall↗

arXiv2025

Wilson Line and Disorder Invariants of Topological One-Dimensional Multiband Models

Topological invariants, such as the winding number, the Chern number, and the Zak phase, characterize the topological phases of bulk materials. Through the bulk-boundary correspondence, these topological phases have a one-to-one correspondence to topological edge states, which are robust to certain classes of disorder. For simple models like the Su-Schrieffer-Heeger (SSH) model, the computation of the winding number and Zak phase are straightforward, however, in multiband systems, this is no longer the case. In this work, we introduce the unwrapped Wilson line across the Brillouin zone to compute the bulk topological invariant. This method can efficiently be implemented numerically to evaluate multiband SSH-type models, including models that have a large number of distinct topological phases. This approach accurately captures all topological edge states, including those overlooked by traditional invariants, such as the winding number and Zak phase. To make a connection to experiments, we determine the sign of the topological invariant by considering a Hall-like configuration. We further introduce different classes of disorder that leave certain edge states protected, while suppressing other edge states, depending on their symmetry properties. Our approach is illustrated using different one-dimensional models, providing a robust framework for understanding topological properties in one-dimensional systems.

cond-mat.dis-nn↗

arXiv2023

Quantum thermometry with single molecules in portable nanoprobes

Understanding heat transport is relevant to develop efficient strategies for thermal management in microelectronics for instance, as well as for fundamental science purposes. However, measuring temperatures in nanostructured environments and in cryogenic conditions remains a challenging task, that requires both high sentitivity and a non-invasive approach. Here we present a portable nanothermometer based on a molecular two-level quantum system that operates in the 3 - 30 K temperature range, with excellent temperature and spatial resolutions on the order of mK and $μ$m, respectively. We validate the performance of this molecular thermometer on nanostructures, by estimating the thermal conductivity of a patterned silicon membrane. In addition, we demonstrate the two-dimensional temperature mapping of a patterned surface via the simultaneous spectroscopy of all thermometers deposited on a sample. These results demonstrate the potential of this molecular thermometer to explore thermal properties and related phenomena at cryogenic temperatures.

physics.app-ph↗

arXiv2018

Phase Diagram of Quantum Hall Breakdown and Non-linear Phenomena for InGaAs/InP Quantum Wells

We investigate non-linear magneto-transport in a Hall bar device made from a strained InGaAs/InP quantum well: a material system with attractive spintronic properties. From extensive maps of the longitudinal differential resistance (r_xx) as a function of current and magnetic (B-) field phase diagrams are generated for quantum Hall breakdown in the strong quantum Hall regime reaching filling factor $ν$=1. By careful illumination the electron sheet density (n) is incremented in small steps and this provides insight into how the transport characteristics evolve with n. We explore in depth the energetics of integer quantum Hall breakdown and provide a simple picture for the principal features in the r_xx maps. A simple tunneling model that captures a number of the characteristic features is introduced. Parameters such as critical Hall electric fields and the exchange-enhanced g-factors for odd-filling factors including nu=1 are extracted. A detailed examination is made of the B-field dependence of the critical current as determined by two different methods and compiled for different values of n. A simple rescaling procedure that allows the critical current data points obtained from r_xx maxima for even-filling to collapse on to a single curve is demonstrated. Exchange-enhanced g-factors for odd-filling are extracted from the compiled data and are compared to those determined by conventional thermal activation measurements. The exchange-enhanced g-factor is found to increase with decreasing n.

cond-mat.dis-nn↗

arXiv2014

Absence of hyperfine effects in $^{13}$C-graphene spin valve devices

The carbon isotope $^{13}$C, in contrast to $^{12}$C, possesses a nuclear magnetic moment and can induce electron spin dephasing in graphene. This effect is usually neglected due to the low abundance of $^{13}$C in natural carbon allotropes ($\sim$1 %). Chemical vapor deposition (CVD) allows for artificial synthesis of graphene solely from a $^{13}$C precursor, potentially amplifying the influence of the nuclear magnetic moments. In this work we study the effect of hyperfine interactions in pure $^{13}$C-graphene on its spin transport properties. Using Hanle precession measurements we determine the spin relaxation time and observe a weak increase of $τ_{s}$ with doping and a weak change of $τ_{s}$ with temperature, as in natural graphene. For comparison we study spin transport in pure $^{12}$C-graphene, also synthesized by CVD, and observe similar spin relaxation properties. As the signatures of hyperfine effects can be better resolved in oblique spin-valve and Hanle configurations, we use finite-element modeling to emulate oblique signals in the presence of a hyperfine magnetic field for typical graphene properties. Unlike in the case of GaAs, hyperfine interactions with $^{13}$C nuclei influence electron spin transport only very weakly, even for a fully polarized nuclear system. Also, in the measurements of the oblique spin-valve and Hanle effects no hyperfine features could be resolved. This work experimentally confirms the weak character of hyperfine interactions and the negligible role of $^{13}$C atoms in the spin dephasing processes in graphene.

cond-mat.mes-hall↗

arXiv2013

Localisation and finite-size effects in graphene flakes

We show that electron states in disordered graphene, with an onsite potential that induces inter-valley scattering, are localised for all energies at disorder as small as 1/6 of the band width of clean graphene. We clarify that, in order for this Anderson-type localisation to be manifested, graphene flakes of size of approximately 200 x 200 nm^2 or larger are needed. For smaller samples, due to the surprisingly large extent of the electronic wave functions, a regime of apparently extended (or even critical) states is identified. Our results complement earlier studies of macroscopically large samples and can explain the divergence of results for finite-size graphene flakes.

cond-mat.mes-hall↗

arXiv2013

Student Variability in Learning Advanced Physics

Learning of advanced physics, requires a combination of empirical, conceptual and theoretical understanding. Students use a combination of these approaches to learn new material. Each student has different prior knowledge and will master new material at a different pace. However, conventional classroom teaching usually does not accommodate the different learning paces of students. To both, study and address this issue, we developed an iterative Online Learning Machine (iOLM), which provides new learning content to each student based on their individual learning pace and tracks their progress individually. The iOLM learning module was implemented using server side web software (php) to supplement the undergraduate course in electromagnetic waves for majors in physics in their second year. This approach follows the hybrid online learning model. Students had to complete a section of the course using iOLM, which was only presented online. The data obtained for this class showed a wide spread of learning paces, ranging from 0.1 to 0.5, where 1 is the maximum pace allowed by iOLM and 0 the lowest. The mean was mu=0.25, with a standard deviation of sigma=0.12. While the pretest showed a positive correlation between the student's pace and performance, the postest had zero correlation, indicating that giving more time and content to weaker students allows them to catch up.

physics.ed-ph↗

arXiv2012

Weak Localization in Graphene: Theory, Simulations and Experiments

We provide a comprehensive picture of magnetotransport in graphene monolayers in the limit of non-quantizing magnetic fields. We discuss the effects of two carrier transport, weak localization, weak anti-localization, and strong localization for graphene devices of various mobilities, through theory, experiments and numerical simulations. In particular, we observe the weak localization of the localization length, which allows us to make the connection between weak and strong localization. It provides a unified framework for both localizations, which explains the observed experimental features. We compare these results to numerical simulation and find a remarkable agreement between theory, experiment and numerics. Various graphene devices were used in this study, including graphene on different substrates, such as glass and silicon, as well as low and high mobility devices.

cond-mat.mes-hall↗

arXiv2010

Low Temperature Electrostatic Force Microscopy of a Deep Two Dimensional Electron Gas using a Quartz Tuning Fork

Using an ultra-low temperature, high magnetic field scanning probe microscope, we have measured electric potentials of a deeply buried two dimensional electron gas (2DEG). Relying on the capacitive coupling between the 2DEG and a resonant tip/cantilever structure, we can extract electrostatic potential information of the 2DEG from the dynamics of the oscillator. We present measurements using a quartz tuning fork oscillator and a 2DEG with a cleaved edge overgrowth structure. The sensitivity of the quartz tuning fork as force sensor is demonstrated by observation of Shubnikov de Haas oscillations at a large tip-2DEG separation distance of more than 500 nm.

cond-mat.mes-hall↗

arXiv2009

Metallic-to-insulating transition in disordered graphene monolayers

We show that when graphene monolayers are disordered, the conductance exhibits a metallic-to-insulating transition, which opens the door to new electronic devices. The transition can be observed by driving the density or Fermi energy through the mobility edge. At the Dirac point the system is localized, whereas at higher densities there is a region of metallic behavior before the system becomes insulating again at higher densities. The region of metallic behavior depends on the disorder strength and eventually vanishes at high disorder. This result is quite unexpected since in square lattices, scaling theory predicts that this metallic region does not exist in two dimensions, in contrast to graphene, where the lattice is a honeycomb.

cond-mat.dis-nn↗

arXiv2009

Cellulose-Bound Magnesium Diboride Superconductivity

Two-phase superconductor tapes were produced by blending high purity magnesium diboride powder with a liquid ethylcellulose-based polymeric binder. This procedure produced a material which is easily formable with a high superconducting transition temperature (38K). We show that the bulk superconducting properties are not affected by the presence of the binder, nor is there any evidence of a chemical reaction between the superconducting particles and the binder. However, the transport properties of the material are strongly affected by the presence of the binder, which leads to a seven order of magnitude increase of the normal state resistance along with a seven order of magnitude decrease of the transport critical current density. This new material is shown to be equivalent to a system of coupled Josephson junctions.

cond-mat.supr-con↗

arXiv2009

The strengthening of reentrant pinning by collective interactions in the peak effect

Since it was first observed about 40 years ago [1], the peak effect has been the subject of numerous research mainly impelled by the desire to determine its exact mechanisms. Despite these efforts, a consensus on this question has yet to be reached. Experimentally, the peak effect indicates a transition from a depinned vortex phase to a reentrant pinning phase at high magnetic field. To study the effects of intrinsic pinning on the peak effect, we consider Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ superconducting metallic glasses in which the vortex pinning force varies depending on the Fe content and in which a huge peak effect is seen as a function of magnetic field. The results are mapped out as a phase diagram in which it is readily seen that the peak effect becomes broader with decreasing pinning force. Typically, pinning can be understood by increased pinning centers, but here, we show that reentrant pinning is due to the strengthening of interactions (while decreasing pinning strength). Our results demonstrate the strengthening of the peak effect by collective effects.

cond-mat.supr-con↗

arXiv2008

Seeing Anderson Localization

Anderson localization was discovered 50 years ago to describe the propagation of electrons in the presence of disorder. The main prediction back then, was the existence of disorder induced localized states, which do not conduct electricity. Many years later it turns out, that the concept of Anderson localization is much more general and applies to almost any type of propagation in time or space, when more than one parameter is relevant (like phase and amplitude). Here we propose a new optical scheme to literally see Anderson localization by varying the optical wavelength or angle of incidence to tune between localized and delocalized states. The occurrence of Anderson localization in the propagation of light, in particular, has become the focus of tremendous interest due to the emergence of new optical technologies and media, such as low dimensional and disordered optical lattices. While several experiments have reported the measurement of Anderson localization of light, many of the observations remain controversial because the effects of absorption and localization have a similar signature, i.e., exponential decrease of the transmission with the system size. In this work, we discuss a system, where we can clearly differentiate between absorption and localization effects because this system is equivalent to a perfect filter, only in the absence of any absorption. Indeed, only one wavelength is perfectly transmitted and all others are fully localized. These results were obtained by developing a new theoretical framework for the average optical transmission through disordered media.

cond-mat.dis-nn↗

arXiv2008

Superconductivity and short range order in metallic glasses Fe$_{x}$Ni$_{1-x}$Zr$_{2}$

In amorphous superconductors, superconducting and vortex pinning properties are strongly linked to the absence of long range order. Consequently, superconductivity and vortex phases can be studied to probe the underlying microstructure and order of the material. This is done here from resistance and local magnetization measurements in the superconducting state of Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ metallic glasses with $0\leq x \leq 0.6$. Firstly, we present typical superconducting properties such as the critical temperature and fields and their dependence on Fe content in these alloys. Then, the observations of peculiar clockwise hysteresis loops, wide double-step transitions and large magnetization fluctuations in glasses containing a large amount of Fe are analyzed to reveal a change in short range order with Fe content.

cond-mat.supr-con↗

arXiv2008

Local magnetization fluctuations in superconducting glasses resolved by Hall sensors

We report on magnetization measurements performed on a series of Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ superconducting metallic glasses with $0\leq x\leq0.5$ using the Hall effect of a nearby 2-dimensional electron gas (2DEG) in a GaAs/Al$_{0.33}$Ga$_{0.67}$As heterostructure as a local probe. The great sensitivity of the Hall effect of the 2DEG in such heterostructure is exploited to determine the magnetization of the superconductor due to the Meissner effect and flux trapping. The data is used to determine the lower critical field B$_{c1}$ of the superconductors as a function of temperature. Surprisingly large fluctuations in the magnetization are also observed and attributed to the presence of large flux clusters in the superconductor.

cond-mat.supr-con↗

arXiv2008

Transverse depinning in weakly-pinned vortices

We present experiments on weakly-pinned vortices, which exhibit a large critical transverse depinning force. These results are obtained in the superconducting metallic glasses Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ using crossed ac and dc driving currents. We study the vortex depinning force due to the transverse ac drive as a function of a longitudinal dc drive; the ac/dc combination permits the separation of the transverse drive from the longitudinal one. We show that the force required for depinning in the transverse direction is greatly enhanced by the longitudinal drive, which demonstrates the existence of a large transverse critical force. The measurements are performed as a function of magnetic field and temperature and show that the transverse critical force exists in a large portion of the phase diagram. Hysteresis observed at the transverse depinning threshold is consistent with a first-order transverse depinning transition.

cond-mat.supr-con↗

arXiv2007

Ensemble Averaged Conductance Fluctuations in Anderson Localized Systems

We demonstrate the presence of energy dependent fluctuations in the localization length, which depend on the disorder distribution. These fluctuations lead to Ensemble Averaged Conductance Fluctuations (EACF) and are enhanced by large disorder. For the binary distribution the fluctuations are strongly enhanced in comparison to the Gaussian and uniform distributions. These results have important implications on ensemble averaged quantities, such as the transmission through quantum wires, where fluctuations can subsist to very high temperatures. For the non-fluctuating part of the localization length in one dimension we obtained an improved analytical expression valid for all disorder strengths by averaging the probability density.

cond-mat.dis-nn↗

arXiv2007

Frequency quenching of microwave induced resistance oscillations in a high mobility two-dimensional electron gas

The frequency dependence of microwave-induced resistance oscillations (MIROs) has been studied experimentally in high-mobility electron GaAs/AlGaAs structures to explore the limits at which these oscillations can be observed. It is found that in dc transport experiments at frequencies above 120 GHz, MIROs start to quench, while above 230 GHz, they completely disappear. The results will need to be understood theoretically but are qualitatively discussed within a model in which forced electronic charge oscillations (plasmons) play an intermediate role in the interaction process between the radiation and the single-particle electron excitations between Landau levels.

cond-mat.mes-hall↗