arXiv subjects

C. Beekman

Publications and source records attributed to C. Beekman.

10 recordsLinked to original sources

arXiv2018

Measurement of transverse wakefields induced by a misaligned positron bunch in a hollow channel plasma accelerator

Hollow channel plasma wakefield acceleration is a proposed method to provide high acceleration gradients for electrons and positrons alike: a key to future lepton colliders. However, beams which are misaligned from the channel axis induce strong transverse wakefields, deflecting beams and reducing the collider luminosity. This undesirable consequence sets a tight constraint on the alignment accuracy of the beam propagating through the channel. Direct measurements of beam misalignment-induced transverse wakefields are therefore essential for designing mitigation strategies. We present the first quantitative measurements of transverse wakefields in a hollow plasma channel, induced by an off-axis 20 GeV positron bunch, and measured with another 20 GeV lower charge trailing positron probe bunch. The measurements are largely consistent with theory.

physics.acc-ph↗

arXiv2014

Strain driven anisotropic magnetoresistance in antiferromagnetic La$_{0.4}$Sr$_{0.6}$MnO$_{3}$

We investigate the effects of strain on antiferromagntic (AFM) single crystal thin films of La$_{1-x}$Sr$_{x}$MnO$_{3}$ (x = 0.6). Nominally unstrained samples have strong magnetoresistance with anisotropic magnetoresistances (AMR) of up to 8%. Compressive strain suppresses magnetoresistance but generates AMR values of up to 63%. Tensile strain presents the only case of a metal-insulator transition and demonstrates a previously unreported AMR behavior. In all three cases, we find evidence of magnetic ordering and no indication of a global ferromagnetic phase transition. These behaviors are attributed to epitaxy induced changes in orbital occupation driving different magnetic ordering types. Our findings suggest that different AFM ordering types have a profound impact on the AMR magnitude and character.

cond-mat.str-el↗

arXiv2013

Unit cell orientation of tetragonal-like BiFeO$_3$ thin films grown on highly miscut LaAlO$_3$ substrates

Synchrotron and lab-scale x-ray diffraction shows that tetragonal-like T'-BiFeO3 films on miscut LaAlO3 substrates (miscut < 5 deg) exhibit (00l)-planes tilted away from those of the substrate as predicted by the "Nagai model" (except for miscut <0.2 deg). Tilts as large as 1 deg are achieved even in 100 nm thick films, strikingly larger than those observed in other perovskites. We attribute this to the large c/a ratio and the high crystalline coherency of the T'-BiFeO3/LaAlO3 interface. This coherency is possible through an observed "diagonal-on-diagonal" alignment. Interestingly, the substrate miscut does not influence the relative population of monoclinic domains.

cond-mat.mtrl-sci↗

arXiv2012

Raman study of the phonon symmetries in BiFeO$_3$ single crystals

In bismuth ferrite (BiFeO3), antiferromagnetic and ferroelectric order coexist at room temperature, making it of particular interest for studying magnetoelectric coupling. The mutual control of magnetic and electric properties is very useful for a wide variety of applications. This has led to an enormous amount of research into the properties of BiFeO$_3$. Nonetheless, one of the most fundamental aspects of this material, namely the symmetries of the lattice vibrations, remains controversial.We present a comprehensive Raman study of BiFeO$_3$ single crystals with the approach of monitoring the Raman spectra while rotating the polarization direction of the excitation laser. Our method results in unambiguous assignment of the phonon symmetries and explains the origin of the controversy in the literature. Furthermore, it provides access to the Raman tensor elements enabling direct comparison with theoretical calculations. Hence, this allows the study of symmetry breaking and coupling mechanisms in a wide range of complex materials and may lead to a noninvasive, all-optical method to determine the orientation and magnitude of the ferroelectric polarization.

cond-mat.mtrl-sci↗

arXiv2012

The role of stepped surfaces on the magnetotransport in strained thin films of La0.67Ca0.33MnO3

We report a comparative study of the properties of very thin films of La0.67Ca0.33MnO3 grown epitaxially under strain on flat SrTiO3 (STO) and on 1 deg miscut STO. For the films on flat STO the transport properties show well-known behavior, namely values of the metal-insulator transition temperature which are strongly reduced with respect to the bulk value. The reduction in films on miscut STO is significantly less strong than in films on flat STO, even though they appear similar as to strain state. Moreover, the residual resistance at low temperatures is lower than for the case of flat films. Magnetically, we find reduced values of the saturation magnetization with respect to the bulk value, indicating the presence of a dead layer in both cases. We argue that the higher density of the step edges on the miscut substrates lead to strain relaxation in the form of point defects and an increased electronic bandwidth, which actually make the electronic properties more robust.

cond-mat.mtrl-sci↗

arXiv2011

Large electric field effects on the resistance of La$_{0.67}$Ca$_{0.33}$MnO$_3$ microstructures

We investigate electric field effects in thin film microbridges of La$_{0.7}$Ca$_{0.3}$MnO$_3$ with the focus on the regime of metal-insulator transition. A mechanically milled SrTiO$_3$ substrate is used as a backgate dielectric. Inside the metal-insulator transition we find a strong unipolar field-induced reduction in resistance, as well as a suppression of the nonlinear features in the I-V curves we observed earlier. We associate the observed effects with a phase separated state in which metallic regions coexist with short range correlated polaron regions. When the glassy polaron phase has fully developed, and closes off the microbridge, the field effects disappear leaving the strongly nonlinear behavior of the transport current unaltered.

cond-mat.mes-hall↗

arXiv2011

Fabrication and Characterization of Topological Insulator Bi$_2$Se$_3$ Nanocrystals

In the recently discovered class of materials known as topological insulators, the presence of strong spin-orbit coupling causes certain topological invariants in the bulk to differ from their values in vacuum. The sudden change of invariants at the interface results in metallic, time reversal invariant surface states whose properties are useful for applications in spintronics and quantum computation. However, a key challenge is to fabricate these materials on the nanoscale appropriate for devices and probing the surface. To this end we have produced 2 nm thick nanocrystals of the topological insulator Bi$_2$Se$_3$ via mechanical exfoliation. For crystals thinner than 10 nm we observe the emergence of an additional mode in the Raman spectrum. The emergent mode intensity together with the other results presented here provide a recipe for production and thickness characterization of Bi$_2$Se$_3$ nanocrystals.

cond-mat.mtrl-sci↗

arXiv2011

Manganese valence and magnetotransport in ultrathin films of La$_{0.67}$Ca$_{0.33}$MnO$_3$

We report a comparative study of the properties of very thin films of La$_{0.67}$Ca$_{0.33}$MnO$_3$ grown epitaxially under strain on flat SrTiO$_3$ (STO), lattice matched on NdGaO$_3$ (NGO), and strained on $1^{\circ}$-miscut STO. We use transmission electron microscopy and electron energy loss spectroscopy to study the microstructure, composition, and Mn valence state. Near the interface we find no significant segregation, but a charge compensation layer where the valence is enhanced over the nominal value of 3.3+, and a relaxation to this value over several nanometer. The transport properties show well-known behavior for the films on flat STO and NGO, namely values of the metal-insulator transition temperature which are strongly (STO) or only little (NGO) reduced with respect to the bulk value. The reduction in films on miscut STO however is less strong than in films on flat STO, even though they appear similar as to strain state and interface layer. Magnetically, we find reduced values of the saturation magnetization for the strained films with respect to the bulk value, which cannot only be ascribed to the interface layer.

cond-mat.mes-hall↗

arXiv2010

Mesoscopic transport in ultrathin films of La$_{0.67}$Ca$_{0.33}$MnO$_3$

We investigate the electrical transport in mesoscopic structures of La$_{0.67}$Ca$_{0.33}$MnO$_3$ in the regime of the metal-insulator transition by fabricating microbridges from strained and unstrained thin films. We measure current-voltage characteristics as function of temperature and in high magnetic fields and with varying film thickness. For strained films, in warming from the metallic to the insulating state, we find non-linear effects in the steep part of the transition characterized by a differential resistance with a strong peak around zero applied current, and saturating at higher currents after resistance drops up to 60 %. We propose that this nonlinear behavior is associated with melting of the insulating state by injecting charge carriers, signalling the occurrence of an intervening phase which involves the formation of short range polaron correlations.

cond-mat.str-el↗

arXiv2007

Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3

We have investigated the electrical transport properties of 8 nm thick La0.67Ca0.33MnO3 films, sputter-deposited on SrTiO3 (STO), and etched into 5 micrometer-wide bridges by Ar-ion etching. We find that even slight overetching of the film leads to conductance of the STO substrate, and asymmetric and non-linear current-voltage (I-V) characteristics. However, a brief oxygen plasma etch allows full recovery of the insulating character of the substrate. The I-V characteristics of the bridges are then fully linear over a large range of current densities. We find colossal magnetoresistance properties typical for strained LCMO on STO but no signature of non-linear effects (so-called electroresistance) connected to electronic inhomogeneites. In the metallic state below 150 K, the highest current densities lead to heating effects and non-linear I-V characteristics.

cond-mat.mtrl-sci↗