208 research outputs found
Resonant electron-lattice cooling in graphene
Controlling energy flows in solids through switchable electron-lattice
cooling can grant access to a range of interesting and potentially useful
energy transport phenomena. Here we discuss a unique switchable
electron-lattice cooling mechanism arising in graphene due to phonon emission
mediated by resonant scattering on defects in crystal lattice, which displays
interesting analogy to the Purcell effect in optics. This mechanism strongly
enhances the electron-phonon cooling rate, since non-equilibrium carriers in
the presence of momentum recoil due to disorder can access a larger phonon
phase space and emit phonons more effciently. Resonant energy dependence of
phonon emission translates into gate-tunable cooling rates, exhibiting giant
enhancement of cooling occurring when the carrier energy is aligned with the
electron resonance of the defect
Imaging resonant dissipation from individual atomic defects in graphene
Conversion of electric current into heat involves microscopic processes that
operate on nanometer length-scales and release minute amounts of power. While
central to our understanding of the electrical properties of materials,
individual mediators of energy dissipation have so far eluded direct
observation. Using scanning nano-thermometry with sub-micro K sensitivity we
visualize and control phonon emission from individual atomic defects in
graphene. The inferred electron-phonon 'cooling power spectrum' exhibits sharp
peaks when the Fermi level comes into resonance with electronic quasi-bound
states at such defects, a hitherto uncharted process. Rare in the bulk but
abundant at graphene's edges, switchable atomic-scale phonon emitters define
the dominant dissipation mechanism. Our work offers new insights for addressing
key materials challenges in modern electronics and engineering dissipation at
the nanoscale
Electrically tunable multi-terminal SQUID-on-tip
We present a new nanoscale superconducting quantum interference device
(SQUID) whose interference pattern can be shifted electrically in-situ. The
device consists of a nanoscale four-terminal/four-junction SQUID fabricated at
the apex of a sharp pipette using a self-aligned three-step deposition of Pb.
In contrast to conventional two-terminal/two-junction SQUIDs that display
optimal sensitivity when flux biased to about a quarter of the flux quantum,
the additional terminals and junctions allow optimal sensitivity at arbitrary
applied flux, thus eliminating the magnetic field "blind spots". We demonstrate
spin sensitivity of 5 to 8 over a continuous field
range of 0 to 0.5 T, with promising applications for nanoscale scanning
magnetic imaging
Multi-layered atomic relaxation in van der Waals heterostructures
When two-dimensional van der Waals materials are stacked to build heterostructures, moir\'e patterns emerge from twisted interfaces or from mismatch in lattice constant of individual layers. Relaxation of the atomic positions is a direct, generic consequence of the moir\'e pattern, with many implications for the physical properties. Moir\'e driven atomic relaxation may be naively thought to be restricted to the interfacial layers and thus irrelevant for multi-layered heterostructures. However, we provide experimental evidence for the importance of the three dimensional nature of the relaxation in two types of van der Waals heterostructures: First, in multi-layer graphene twisted on graphite at small twist angles () we observe propagation of relaxation domains even beyond 18 graphene layers. Second, we show how for multi-layer PdTe on BiSe the moir\'e lattice constant depends on the number of PdTe layers. Motivated by the experimental findings, we developed a continuum approach to model multi-layered relaxation processes based on the generalized stacking fault energy functional given by ab-initio simulations. Leveraging the continuum property of the approach enables us to access large scale regimes and achieve agreement with our experimental data for both systems. Furthermore it is well known that the electronic structure of graphene sensitively depends on local lattice deformations. Therefore we study the impact of multi-layered relaxation on the local density of states of the twisted graphitic system. We identify measurable implications for the system, experimentally accessible by scanning tunneling microscopy. Our multi-layered relaxation approach is not restricted to the discussed systems, and can be used to uncover the impact of an interfacial defect on various layered systems of interest
The Animalistic Gullet and the Godlike Soul: Reframing Sacrifice in Midrash Leviticus Rabbah
This article proposes an analysis of two homiletic units in the Palestinian Midrash Leviticus Rabbah, which revolve around biblical chapters pertaining to sacrifices. A theme that pervades these units is that of eating as an animalistic activity that often entails moral depravity. In contrast, the act of sacrificing is constructed in these units as one in which one is willing to give up one's own nourishment, and in a sense one's own “soul,” in order to offer it to God. Many of the motifs used to vilify eating in the Midrash can be traced in moralistic Greek, Roman, and early Christian diatribes preaching for moderation in eating or for asceticism; the homilists in Leviticus Rabbah, however, utilize these popular motifs in order to present sacrifice as the spiritual contrary of eating, and thus to give the obsolete practice of sacrifice cultural cachet and compelling meanings
Probing dynamics and pinning of single vortices in superconductors at nanometer scales
The dynamics of quantized magnetic vortices and their pinning by materials
defects determine electromagnetic properties of superconductors, particularly
their ability to carry non-dissipative currents. Despite recent advances in the
understanding of the complex physics of vortex matter, the behavior of vortices
driven by current through a multi-scale potential of the actual materials
defects is still not well understood, mostly due to the scarcity of appropriate
experimental tools capable of tracing vortex trajectories on nanometer scales.
Using a novel scanning superconducting quantum interference microscope we
report here an investigation of controlled dynamics of vortices in lead films
with sub-Angstrom spatial resolution and unprecedented sensitivity. We
measured, for the first time, the fundamental dependence of the elementary
pinning force of multiple defects on the vortex displacement, revealing a far
more complex behavior than has previously been recognized, including striking
spring softening and broken-spring depinning, as well as spontaneous hysteretic
switching between cellular vortex trajectories. Our results indicate the
importance of thermal fluctuations even at 4.2 K and of the vital role of
ripples in the pinning potential, giving new insights into the mechanisms of
magnetic relaxation and electromagnetic response of superconductors.Comment: 15 pages and 5 figures (main text) + 15 pages and 11 figures
(supplementary material
Unconventional non-local relaxation dynamics in a twisted trilayer graphene moiré superlattice
The electronic and structural properties of atomically thin materials can be controllably tuned by assembling them with an interlayer twist. During this process, constituent layers spontaneously rearrange themselves in search of a lowest energy configuration. Such relaxation phenomena can lead to unexpected and novel material properties. Here, we study twisted double trilayer graphene (TDTG) using nano-optical and tunneling spectroscopy tools. We reveal a surprising optical and electronic contrast, as well as a stacking energy imbalance emerging between the moiré domains. We attribute this contrast to an unconventional form of lattice relaxation in which an entire graphene layer spontaneously shifts position during assembly, resulting in domains of ABABAB and BCBACA stacking. We analyze the energetics of this transition and demonstrate that it is the result of a non-local relaxation process, in which an energy gain in one domain of the moiré lattice is paid for by a relaxation that occurs in the other
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