High quality thin films of the ferromagnetic semiconductor EuO have been prepared and were studied using a new form of spin-resolved spectroscopy. We observed large changes in the electronic structure across the Curie and metal-insulator transition temperature. We found that these are caused by the exchange splitting of the conduction band in the ferromagnetic state, which is as large as 0.6 eV. We also present strong evidence that the bottom of the conduction band consists mainly of majority spins. This implies that doped charge carriers in EuO are practically fully spin polarized.EuO is a semiconductor with a band gap of about 1.2 eV and is one of the very rare ferromagnetic oxides [1,2]. Its Curie temperature (T c ) is around 69 K and the crystal structure is rocksalt (fcc) with a lattice constant of 5.144Å. Eu-rich EuO becomes metallic below T c and the metal-insulator transition (MIT) is spectacular: the resistivity drops by as much as 8 orders of magnitude [3,4]. Moreover, an applied magnetic field shifts the MIT temperature considerably, resulting in a colossal magnetoresistance (CMR) with changes in resistivity of up to 6 orders of magnitude [4]. This CMR behavior in EuO is in fact more extreme than in the now much investigated La 1−x Sr x MnO 3 materials [5,6]. Much what is known about the basic electronic structure of EuO dates back to about 30 years ago and is based mainly on optical measurements [7][8][9] and band structure calculations [10]. With the properties being so spectacular, it is surprising that very little has been done so far to determine the electronic structure of EuO using more modern and direct methods like electron spectroscopies.Here we introduce spin-resolved x-ray absorption spectroscopy, a new type of spin-resolved electron spectroscopy technique to study directly the conduction band of EuO where most of the effects related to the MIT and CMR are expected to show up. Spin-resolved measurements of the conduction band could previously only be obtained by spin-polarized inverse photoemission spectroscopy. Spin-resolved x-ray absorption spectroscopy is an alternative technique which is especially well suited to study ferromagnetic oxides, a currently interesting broad class of materials. Using this technique we observed large changes in the conduction band across T c and we were able to show experimentally that these are caused by an exchange splitting of the conduction band below T c . Moreover, we found that this splitting is as large as 0.6 eV and show that the states close to the bottom of the conduction band are almost fully spin-polarized, which is very interesting for basic research in the field of spintronics.The experiments were performed using the helical undulator [11] based beamline ID12B [12] at the European Synchrotron Radiation Facility (ESRF) in Grenoble. Photoemission and Auger spectra were recorded using a 140 mm mean radius hemispherical analyzer coupled to a mini-Mott 25 kV spin polarimeter [13]. The spin detector had an efficiency (Sherman function) of 17%, and t...

The operating principle of squeeze-film pressure sensors is based on the pressure dependence of a membrane's resonance frequency, caused by the compression of the surrounding gas which changes the resonator stiffness. To realize such sensors, not only strong and flexible membranes are required, but also minimization of the membrane's mass is essential to maximize responsivity. Here, we demonstrate the use of a few-layer graphene membrane as a squeeze-film pressure sensor. A clear pressure dependence of the membrane's resonant frequency is observed, with a frequency shift of 4 MHz between 8 and 1000 mbar. The sensor shows a reproducible response and no hysteresis. The measured responsivity of the device is 9000 Hz/mbar, which is a factor 45 higher than state-of-the-art MEMS-based squeeze-film pressure sensors while using a 25 times smaller membrane area.

Heat engines provide most of our mechanical power and are essential for transportation on macroscopic scale. However, although significant progress has been made in the miniaturization of electrostatic engines, it has proven difficult to reduce the size of liquid or gas driven heat engines below 10^7 um^3. Here we demonstrate that a crystalline silicon structure operates as a cyclic piezoresistive heat engine when it is driven by a sufficiently high DC current. A 0.34 um^3 engine beam draws heat from the DC current using the piezoresistive effect and converts it into mechanical work by expansion and contraction at different temperatures. This mechanical power drives a silicon resonator of 1.1x10^3 um^3 into sustained oscillation. Even below the oscillation threshold the engine beam continues to amplify the resonator's Brownian motion. When its thermodynamic cycle is inverted, the structure is shown to reduce these thermal fluctuations, therefore operating as a refrigerator.Comment: Updated version after publication to make it almost identical to the Nature Physics article. During the review process the preprint v1 was merged with part of the results from arXiv:0904.3748 (please check this manuscript for more details on the measurements and simulations

Membranes of suspended two-dimensional materials show a large variability in mechanical properties, in part due to static and dynamic wrinkles. As a consequence, experiments typically show a multitude of nanomechanical resonance peaks, which make an unambiguous identification of the vibrational modes difficult. Here, we probe the motion of graphene nanodrum resonators with spatial resolution using a phase-sensitive interferometer. By simultaneously visualizing the local phase and amplitude of the driven motion, we show that unexplained spectral features represent split degenerate modes. When taking these into account, the resonance frequencies up to the eighth vibrational mode agree with theory. The corresponding displacement profiles, however, are remarkably different from theory, as small imperfections increasingly deform the nodal lines for the higher modes. The Brownian motion, which is used to calibrate the local displacement, exhibits a similar mode pattern. The experiments clarify the complicated dynamic behavior of suspended two-dimensional materials, which is crucial for reproducible fabrication and applications.

Owing to their atomic-scale thickness, the resonances of two-dimensional (2D) material membranes show signatures of nonlinearities at forces of only a few picoNewtons. Although the linear dynamics of membranes is well understood, the exact relation between the nonlinear response and the resonator’s material properties has remained elusive. Here we show a method for determining the Young’s modulus of suspended 2D material membranes from their nonlinear dynamic response. To demonstrate the method, we perform measurements on graphene and MoS2 nanodrums electrostatically driven into the nonlinear regime at multiple driving forces. We show that a set of frequency response curves can be fitted using only the cubic spring constant as a fit parameter, which we then relate to the Young’s modulus of the material using membrane theory. The presented method is fast, contactless, and provides a platform for high-frequency characterization of the mechanical properties of 2D materials.