Fluidity Onset in Graphene

Viscous electron fluids have emerged recently as a new paradigm of strongly-correlated electron transport in solids. Here we report on a direct observation of the transition to this long-sought-for state of matter in a high-mobility electron system in graphene. Unexpectedly, the electron flow is found to be interaction-dominated but non-hydrodynamic (quasiballistic) in a wide temperature range, showing signatures of viscous flows only at relatively high temperatures. The transition between the two regimes is characterized by a sharp maximum of negative resistance, probed in proximity to the current injector. The resistance decreases as the system goes deeper into the hydrodynamic regime. In a perfect darkness-before-daybreak manner, the interaction-dominated negative response is strongest at the transition to the quasiballistic regime. Our work provides the first demonstration of how the viscous fluid behavior emerges in an interacting electron system.Comment: 8pgs, 4fg

Magnetization Signature of Topological Surface States in a Non-Symmorphic Superconductor

Superconductors with nontrivial band structure topology represent a class of materials with unconventional and potentially useful properties. Recent years have seen much success in creating artificial hybrid structures exhibiting the main characteristics of 2D topological superconductors. Yet, bulk materials known to combine inherent superconductivity with nontrivial topology remain scarce, largely because distinguishing their central characteristic—the topological surface states—has proved challenging due to a dominant contribution from the superconducting bulk. In this work, a highly anomalous behavior of surface superconductivity in topologically nontrivial 3D superconductor In2Bi, where the surface states result from its nontrivial band structure, itself a consequence of the non-symmorphic crystal symmetry and strong spin–orbit coupling, is reported. In contrast to smoothly decreasing diamagnetic susceptibility above the bulk critical field, Hc2, as seen in conventional superconductors, a near-perfect, Meissner-like screening of low-frequency magnetic fields well above Hc2 is observed. The enhanced diamagnetism disappears at a new phase transition close to the critical field of surface superconductivity, Hc3. Using theoretical modeling, the anomalous screening is shown to be consistent with modification of surface superconductivity by the topological surface states. The possibility of detecting signatures of the surface states using macroscopic magnetization provides a new tool for the discovery and identification of topological superconductor

Magnetization Signature of Topological Surface States in a Non-Symmorphic Superconductor

Superconductors with nontrivial band structure topology represent a class of materials with unconventional and potentially useful properties. Recent years have seen much success in creating artificial hybrid structures exhibiting the main characteristics of 2D topological superconductors. Yet, bulk materials known to combine inherent superconductivity with nontrivial topology remain scarce, largely because distinguishing their central characteristic—the topological surface states—has proved challenging due to a dominant contribution from the superconducting bulk. In this work, a highly anomalous behavior of surface superconductivity in topologically nontrivial 3D superconductor In2Bi, where the surface states result from its nontrivial band structure, itself a consequence of the non-symmorphic crystal symmetry and strong spin–orbit coupling, is reported. In contrast to smoothly decreasing diamagnetic susceptibility above the bulk critical field, Hc2, as seen in conventional superconductors, a near-perfect, Meissner-like screening of low-frequency magnetic fields well above Hc2 is observed. The enhanced diamagnetism disappears at a new phase transition close to the critical field of surface superconductivity, Hc3. Using theoretical modeling, the anomalous screening is shown to be consistent with modification of surface superconductivity by the topological surface states. The possibility of detecting signatures of the surface states using macroscopic magnetization provides a new tool for the discovery and identification of topological superconductor

Scanning SQUID-on-tip microscope in a top-loading cryogen-free dilution refrigerator

The scanning superconducting quantum interference device (SQUID) fabricated on the tip of a sharp quartz pipette (SQUID-on-tip) has emerged as a versatile tool for nanoscale imaging of magnetic, thermal, and transport properties of microscopic devices of quantum materials. We present the design and performance of a scanning SQUID-on-tip microscope in a top-loading probe of a cryogen-free dilution refrigerator. The microscope is enclosed in a custom-made vacuum-tight cell mounted at the bottom of the probe and is suspended by springs to suppress vibrations caused by the pulse tube cryocooler. Two capillaries allow in-situ control of helium exchange gas pressure in the cell that is required for thermal imaging. A nanoscale heater is used to create local temperature gradients in the sample, which enables quantitative characterization of the relative vibrations between the tip and the sample. The spectrum of the vibrations shows distinct resonant peaks with maximal power density of about 27 nm/Hz$^{1/2}$ in the in-plane direction. The performance of the SQUID-on-tip microscope is demonstrated by magnetic imaging of the MnBi$_2$Te$_4$ magnetic topological insulator, magnetization and current distribution imaging in a SrRuO$_3$ ferromagnetic oxide thin film, and by thermal imaging of dissipation in graphene.Comment: Submitted to Review of Scientific Instrument

Giant magnetoresistance of Dirac plasma in high-mobility graphene

The most recognizable feature of graphene's electronic spectrum is its Dirac point around which interesting phenomena tend to cluster. At low temperatures, the intrinsic behavior in this regime is often obscured by charge inhomogeneity but thermal excitations can overcome the disorder at elevated temperatures and create electron-hole plasma of Dirac fermions. The Dirac plasma has been found to exhibit unusual properties including quantum critical scattering and hydrodynamic flow. However, little is known about the plasma's behavior in magnetic fields. Here we report magnetotransport in this quantum-critical regime. In low fields, the plasma exhibits giant parabolic magnetoresistivity reaching >100% in 0.1 T even at room temperature. This is orders of magnitude higher than magnetoresistivity found in any other system at such temperatures. We show that this behavior is unique to monolayer graphene, being underpinned by its massless spectrum and ultrahigh mobility, despite frequent (Planckian-limit) scattering. With the onset of Landau quantization in a few T, where the electron-hole plasma resides entirely on the zeroth Landau level, giant linear magnetoresistivity emerges. It is nearly independent of temperature and can be suppressed by proximity screening, indicating a many-body origin. Clear parallels with magnetotransport in strange metals and so-called quantum linear magnetoresistance predicted for Weyl metals offer an interesting playground to further explore relevant physics using this well-defined quantum-critical 2D system.Comment: 8 pages, 3 figure

Magnetization Signature of Topological Surface States in a Non‐Symmorphic Superconductor

From Wiley via Jisc Publications RouterHistory: received 2021-04-28, rev-recd 2021-06-16, pub-electronic 2021-08-08Article version: VoRPublication status: PublishedFunder: European Union's Horizon 2020 research and innovation programme; Grant(s): 785219, 881603Funder: EPSRC; Id: http://dx.doi.org/10.13039/501100000266; Grant(s): EP/L01548XAbstract: Superconductors with nontrivial band structure topology represent a class of materials with unconventional and potentially useful properties. Recent years have seen much success in creating artificial hybrid structures exhibiting the main characteristics of 2D topological superconductors. Yet, bulk materials known to combine inherent superconductivity with nontrivial topology remain scarce, largely because distinguishing their central characteristic—the topological surface states—has proved challenging due to a dominant contribution from the superconducting bulk. In this work, a highly anomalous behavior of surface superconductivity in topologically nontrivial 3D superconductor In2Bi, where the surface states result from its nontrivial band structure, itself a consequence of the non‐symmorphic crystal symmetry and strong spin–orbit coupling, is reported. In contrast to smoothly decreasing diamagnetic susceptibility above the bulk critical field, Hc2, as seen in conventional superconductors, a near‐perfect, Meissner‐like screening of low‐frequency magnetic fields well above Hc2 is observed. The enhanced diamagnetism disappears at a new phase transition close to the critical field of surface superconductivity, Hc3. Using theoretical modeling, the anomalous screening is shown to be consistent with modification of surface superconductivity by the topological surface states. The possibility of detecting signatures of the surface states using macroscopic magnetization provides a new tool for the discovery and identification of topological superconductors

Polarized blazar X-rays imply particle acceleration in shocks

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier1,2,3, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree ΠX of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock

X-ray Polarization Observations of BL Lacertae

Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus the Synchrotron emission to be responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae performed with the Imaging X-ray Polarimetry Explorer ({IXPE}), from which an upper limit to the polarization degree $\Pi_X<$12.6\% was found in the 2-8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.Comment: 17 pages, 5 figures, accepted for publication in ApJ

Solution pH Effect on Drain-Gate Characteristics of SOI FET Biosensor

Nanowire or nanobelt sensors based on silicon-on-insulator field-effect transistors (SOI-FETs) are one of the leading directions of label-free biosensors. An essential issue in this device construction type is obtaining reproducible results from electrochemical measurements. It is affected by many factors, including the measuring solution and the design parameters of the sensor. The biosensor surface should be charged minimally for the highest sensitivity and maximum effect from interaction with other charged molecules. Therefore, the pH value should be chosen so that the surface has a minimum charge. Here, we studied the SOI-FET sensor containing 12 nanobelt elements concatenated on a single substrate. Two types of sensing elements of similar design and different widths (0.2 or 3 &mu;m) were located in the chips. The drain-gate measurements of wires with a width of 3 &micro;m are sufficiently reproducible for the entire chip to obtain measurement statistics in air and deionized water. For the pH values from 3 to 12, we found significant changes in source-drain characteristics of nanobelts, which reach the plateau at pH values of 7 and higher. High pH sensitivity (ca. 1500 and 970 mV/pH) was observed in sensors of 3 &mu;m and 0.2 &mu;m in width in the range of pH values from 3 to 7. We found a higher &ldquo;on&rdquo; current to &ldquo;off&rdquo; current ratio for wide wires. At all studied pH values, Ion/Ioff was up to 4600 and 30,800 for 0.2 and 3 &mu;m wires, respectively. In the scheme on the source-drain current measurements at fixed gate voltages, the highest sensitivity to the pH changes reaches a gate voltage of 13 and 19 V for 0.2 &mu;m and 3 &mu;m sensors, respectively. In summary, the most suitable is 3 &mu;m nanobelt sensing elements for the reliable analysis of biomolecules and measurements at pH over 7