Search

The thermal Hall effect, defined as a heat current response transversal to an applied temperature gradient, is a central experimental probe of exotic electrically insulating phases of matter. A key question is how the interplay between magnetic and structural degrees of freedom gives rise to a nonzero thermal Hall conductivity (THC). Here, we present evidence for an intrinsic thermal Hall effect in the Heisenberg-Kitaev antiferromagnet and spin-liquid candidate Na$_2$Co$_2$TeO$_6$ brought about by the quantum-geometric Berry curvature of so-called magnon polarons, resulting from magnon-phonon hybridization. At low temperatures, our field- and temperature-dependent measurements show a negative THC for magnetic fields below 10 T and a sign change to positive THC above. Theoretically, the sign and the order of magnitude of the THC cannot be solely explained with magnetic excitations. We demonstrate that, by incorporating spin-lattice coupling into our theoretical calculations, the Berry curvature of magnon polarons counteracts the purely magnonic contribution, reverses the overall sign of the THC, and increases its magnitude, which significantly improves agreement with experimental data. Our work highlights the crucial role of spin-lattice coupling in the thermal Hall effect., Comment: 19 pages, 4 figures

Analysis of nanoscale liquids, including wetting and flow phenomena, is a scientific challenge with far reaching implications for industrial technologies. We report the conception, development, and application of an integrated platform for the experimental characterization of liquids at the nanometer scale. The platform combines the sensing functionalities of an integrated, two-dimensional electronic device array with in situ application of highly sensitive optical micro-spectroscopy and atomic force microscopy. We demonstrate the performance capabilities of the platform with an embodiment based on an array of integrated, optically transparent graphene sensors. The application of electronic and optical sensing in the platform allows for differentiating between liquids electronically, for determining a liquid's molecular fingerprint, and for monitoring surface wetting dynamics in real time. In order to explore the platform's sensitivity limits, we record topographies and optical spectra of individual, spatially isolated sessile oil emulsion droplets having volumes of less than ten attoliters. The results demonstrate that integrated measurement functionalities based on two-dimensional materials have the potential to push lab-on-chip based analysis from the microscale to the nanoscale., Comment: 20 pages, 5 figures

The investigation of liquid flow at the nanoscale is a key area of applied research with high relevance to Physics, Chemistry and Biology. We introduce a method and a device that allows to spatially resolve liquid flow by integrating an array of graphene-based magnetic sensors used for tracking the movement of magnetic nanoparticles in the liquid under investigation. With a novel device conception based on standard integration processes and experimentally verified material parameters, we simulate the performance of a single sensor pixel, as well as the whole array, for tracking magnetic nanoparticles. The results demonstrate the ability (a) to detect individual nanoparticles in the liquid and (b) to reconstruct particle trajectories across the sensor array as a function of time, in what we call "Magnetic Nanoparticle Velocimetry" technique. Being a non-optical detection method, potential applications include particle tracking and flow analysis in opaque media at sub-micron scales.

The magnetization reversal of three-segment cylindrical nanoelements comprising of alternating nanowire and nanotube sections is investigated by means of Monte Carlo simulations. Such nanoelements may feature a three-state behaviour with an intermediate plateau in the hysteresis curve due to a metastable pinning of the domain walls at the wire-tube interfaces. It turns out that vortex as well as transverse domain walls contribute to the magnetization reversal. By varying the geometric parameters, the sequence, or the material of the elements the nucleation location of domain walls, as well as their nucleation field, can be tailored. Especially interesting is the novel possibility to drive domain walls coherently in the same or in opposite directions by changing the geometry of the hybrid nanoelement. This important feature adds additional flexibility to the construction of logical devices based on domain wall movement. Another prominent outcome is that domain walls can be nucleated near the centre of the element and then traverse to the outer tips of the cylindrical structure when the applied field is increased, which also opens the possibility to use these three-segment nanoelements for the field induced delivery of domain walls as substitutes for large nucleation pads., Comment: 17 pages (1.5 spacing), 8 Figures, 1 Table

In Microscopic Particle Image Velocimetry ($\mu$PIV), velocity fields in microchannels are sampled over finite volumes within which the velocity fields themselves may vary significantly. In the past, this has limited measurements often to be only qualitative in nature, blind to velocity magnitudes. In the pursuit of quantitatively useful results, one has treated the effects of the finite volume as errors that must be corrected by means of ever more complicated processing techniques. Resulting measurements have limited robustness and require convoluted efforts to understand measurement uncertainties. To increase the simplicity and utility of $\mu$PIV measurements, we introduce a straightforward method, based directly on measurement, by which one can determine the size and shape of the volume over which moving fluids are sampled. By comparing measurements with simulation, we verify that this method enables quantitative measurement of velocity profiles across entire channels, as well as an understanding of experimental uncertainties. We show how the method permits measurement of an unknown flow rate through a channel of known geometry. We demonstrate the method to be robust against common sources of experimental uncertainty. We also apply the theory to model the technique of Scanning $\mu$PIV, which is often used to locate the center of a channel, and we show how and why it can in fact misidentify the center. The results have general implications for research and development that requires reliable, quantitative measurement of fluid flow on the micrometer scale and below., Comment: 20 pages, 10 figures, included OpenPIV reference and website

Using first-principles density-functional calculations, we address the magnetic properties of the ferromagnet/antiferromagnet La(0.67)Sr(0.33)MnO3/BiFeO3(001) heterojunctions, and investigate possible driving mechanisms for a ferromagnetic (FM) interfacial ordering of the Fe spins recently observed experimentally. We find that the chemically abrupt defect-free La(0.67)Sr(0.33)MnO3/BiFeO3(001) heterojunction displays, as ground state, an ordering with compensated Fe spins. Cation Fe/Mn intermixing at the interface tends to favour, instead, a FM interfacial order of the Fe spins, coupled antiferromagnetically to the bulk La(0.67)Sr(0.33)MnO3 spins, as observed experimentally. Such trends are understood based on a model description of the energetics of the exchange interactions., Comment: 6 pages, 6 figures

This paper introduces a new approach for simulating magnetic properties of nanocomposites comprising magnetic particles embedded in a non-magnetic matrix, taking into account the 3D structure of the system in which particles' positions correctly mimic real samples. The proposed approach develops a multistage simulation procedure in which the size and distribution of particles within the hosting matrix is firstly attained by means of the Cell Dynamic System (CDS) model. The 3D structure provided by the CDS step is further employed in a Monte Carlo (MC) simulation of zero-field-cooled/field-cooled (ZFC/FC) and magnetic hysteresis loops ($M \times H$ curves) for the system. Simulations are aimed to draw a realistic picture of the as-produced ultra-thin films comprising maghemite nanoparticles dispersed in polyaniline. Comparison (ZFC/FC and $M \times H$ curves) between experiments and simulations regarding the maximum of the ZFC curve ($T_{\scriptsize MAX}$), remanence ($M_R/M_s$) and coercivity ($H_C$) revealed the great accuracy of the multistage approach proposed here while providing information about the system's morphology and magnetic properties. For a typical sample the value we found experimentally for $T_{\scriptsize MAX}$ (54 K) was very close to the value provided by the simulation (53 K). For the parameters depending on the nanoparticle clustering the experimental values were consistently lower ($M_R/M_s$ = 0.32 and $H_C$ = 210 Oe) than the values we found in the simulation ($M_R/M_s$ = 0.53 and $H_C$ = 274 Oe). Indeed, the approach introduced here is very promising for the design of real magnetic nanocomposite samples with optimized features., Comment: 19 pages (one column), 5 figures

During the last years great attention has been given to the encapsulation of magnetic nanoparticles. In this work we investigated the stability of small magnetic particles inside magnetic nanotubes. Multisegmented nanotubes were tested in order to optimize the stability of the particle inside the nanotubes. Our results evidenced that multisegmented nanotubes are more efficient to entrap the particles at temperatures up to hundreds of kelvins., Comment: 4 pages, 4 figures

A detailed study of the circular photogalvanic effect (CPGE) in SiGe structures is presented. It is shown that the CPGE becomes possible due to the built-in asymmetry of quantum wells (QWs) in compositionally stepped samples and in asymmetrically doped structures. The photocurrent arises due to optical spin orientation of free carriers in QWs with spin splitting in k-space. It is shown that the effect can be applied to probe the macroscopic in-plane symmetry of low dimensional structures and allowing to conclude on Rashba or Dresselhaus terms in the Hamiltonian.