Martin Panholzer

Observation of a roton collective mode in a two-dimensional Fermi liquid

Understanding the dynamics of correlated many-body quantum systems is a challenge for modern physics. Owing to the simplicity of their Hamiltonians, 4He (bosons) and 3He (fermions) have served as model systems for strongly interacting quantum fluids, with substantial efforts devoted to their understanding. An important milestone was the direct observation of the collective phonon–roton mode in liquid 4He by neutron scattering, verifying Landau’s prediction and his fruitful concept of elementary excitations. In a Fermi system, collective density fluctuations (known as ‘zero-sound’ in 3He, and ‘plasmons’ in charged systems) and incoherent particle–hole excitations are observed. At small wavevectors and energies, both types of excitation are described by Landau’s theory of Fermi liquids. At higher wavevectors, the collective mode enters the particle–hole band, where it is strongly damped. The dynamics of Fermi liquids at high wavevectors was thus believed to be essentially incoherent. Here we report inelastic neutron scattering measurements of a monolayer of liquid 3He, observing a roton-like excitation. We find that the collective density mode reappears as a well defined excitation at momentum transfers larger than twice the Fermi momentum. We thus observe unexpected collective behaviour of a Fermi many-body system in the regime beyond the scope of Landau’s theory. A satisfactory interpretation of the measured spectra is obtained using a dynamic many-body theory.

Design of a highly reliable fan with magnetic bearings

Bearing failures are, according to long-term analyses at ebm-papst St. Georgen GmbH & Co. KG, responsible for 90% of all compact fan breakdowns. The working life of a fan can be increased considerably by using a magnetically levitated fan, where the impeller has no contact with the stator. This paper presents the design of a low-cost magnetically levitated fan with passive magnetic bearings (PMBs) to stabilise radial and tilt deflections of the rotor. The application of an optimised viscoelastic support to the stator introduces sufficient damping to the passively stabilised degrees of freedom. The optimisation of the stiffness and damping and the design of the key components, namely the PMB, the active magnetic bearing and the passive damping device is discussed. Finally, the built prototype is presented and the measurement results are analysed.

DYNAMIC PAIR EXCITATIONS IN ALUMINUM

We present a calculation of the excitation spectrum of the electron liquid that includes time-dependent pair correlations. For the charged boson fluid these correlations provide a major mechanism for lowering the plasmon energy; here we extend that study to the much more demanding fermionic case. Based on the formalism of correlated basis functions we derive coupled equations of motion for time-dependent 1- and 2-particle correlation amplitudes. Our solution strategy for these equations ensures the fulfillment of the first two energy–weighted sum rules and, in the appropriate limit, is consistent with the bosonic version. Results are presented for the dynamic structure factor with special emphasis being put on studying the double plasmon.

Simulation and optimization of an eddy current position sensor

Position measurement is a major topic for the control of nearly every moving or rotating device. The demand for reliable and affordable position measurement systems is high. This paper presents the design of an eddy current position sensor and explains the working principle, which is based on injection locking of coupled oscillators. Thereby, the stability and the characteristic of the sensor are dependent on the combination of the used components. To investigate the complex dependency of the parameters, a single-axis sensor was analyzed by a coupled magnetic 2D finite element and an analog electronic simulation. To achieve a stable operation, a high linearity and a high sensitivity an optimization of the sensor was conducted. The suitability of the simulation results were verified by measurements on an implemented sensor.

Two-dimensional Fermi liquids sustain surprising roton-like plasmons beyond the particle-hole band

Using inelastic neutron scattering, we have investigated the elementary excitations of an isotropic two-dimensional Fermi liquid, 3He adsorbed on graphite. We provide in this article a detailed account of the principles and methods which allowed measuring for the first time inelastic spectra on a liquid monolayer of 3He, a strong neutron absorber. We also summarise the results presented at this Conference, and review our recent experimental and theoretical work on this this interacting many-body system. At low wave-vectors, near the edge of the particle-hole band, a mode identified as the zero-sound excitation by comparison to our theoretical calculations, is found as predicted at energies much lower than in bulk 3He. The mode enters the particle-hole band, where it undergoes Landau damping. Surprisingly, however, intensity is observed in the neutron spectra at wave-vectors larger than twice the Fermi wave-vector. This new branch is interpreted as the high wave-vector continuation of the zero-sound mode, in agreement with the theory. The results open new perspectives in the understanding of the dynamics of correlated fermions.

Optimization and realization of a multi-pole permanent magnetic bearing with rotating magnetization

The focus of this paper is on optimizing the stiffness of a permanent magnetic single-ring bearing solely by changing the magnetization pattern. More precisely, using rotating magnetization with two magnetic poles rather than homogeneous magnetization results in a 3.7-fold improvement in stiffness. Usually, rotating magnetization is realized by Halbach stacking, that is, stacking several homogeneously magnetized rings with different directions of magnetization. The approach followed in this work realizes a continuously rotating magnetization pattern using only one ring. Optimization of the magnetization process, as presented in this paper, is crucial to achieving the maximum possible stiffness for the specified bearing dimensions. Furthermore, an analytical stiffness calculation method for bearings with arbitrary magnetization pattern is introduced, and the results are compared with finite-element calculations and measurements.

REEMERGENCE OF THE COLLECTIVE MODE IN 3HE AND ELECTRON LAYERS

Neutron scattering experiments on a 3He layer on graphite show an unexpected behavior of the collective mode. After having been broadened by Landau damping at intermediate wave vectors, the phonon-roton mode resharpens at large wave vectors and even emerges from the particle-hole continuum at low energies. The measured spectra cannot be explained by a random phase approximation with any static interaction. We show here that the data are well described if dynamic two-pair fluctuations are accounted for. We predict similar effects for electron layers.

Double-plasmon excitations in the alkali metals

We calculate the excitation spectrum of the electron liquid using the formalism of correlated basis functions including time-dependent pair correlations. Using the static structure factor of the ground state as sole input, our formalism is naturally suited for studying correlation effects on the energy loss function. The most prominent example is the double plasmon, for which our results are in good agreement with recent experiments on various alkali metals.

Dynamic pair excitations in 2D Fermi fluids

We apply a theory (Bohm et al 2006 AIP Conf. Proc. 850 111, 2007 Int. J. Mod. Phys. B 21 2055) developed recently on dynamic two-pair fluctuations to layered systems, such as electrons in semiconductor hetero-structures or He on graphite. The theory fulfils the zeroth and first frequency moment sum rule. Results are presented for the static effective particle–hole interaction, the dynamic structure function and the dispersion of the plasmon.