Abstract
As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm’s law. Depending on the length scales of momentum conserving (
l
MC
) and ...relaxing (
l
MR
) electron scattering, and the device size (
d
), current flows may shift from ohmic to ballistic to hydrodynamic regimes. So far, an in situ methodology to obtain these parameters within a micro/nanodevice is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain
l
MR
even when
l
MR
≫
d
. We extract
l
MR
from the Sondheimer amplitude in WP
2
, at temperatures up to
T
~ 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on
μ
m-sized devices are in excellent agreement with experimental reports of the bulk
l
MR
and confirm that WP
2
can be microfabricated without degradation. These results conclusively establish Sondheimer oscillations as a quantitative probe of
l
MR
in micro-devices.
We present kinetic Monte Carlo simulations of a modified Yaldram-Khan (YK) model for the catalytic reduction of NO on a surface. The model includes the individual desorption of CO molecules and N ...atoms, associated with temperature effects. We find that the combined desorption of these species produces a reactive phase that is absent in the original YK model on a square lattice. In this reactive phase, N checkerboard structures are reduced in size and reactive zones between them can be established.
The crystal symmetry of a material dictates the type of topological band structure it may host, and therefore, symmetry is the guiding principle to find topological materials. Here we introduce an ...alternative guiding principle, which we call ‘quasi-symmetry’. This is the situation where a Hamiltonian has exact symmetry at a lower order that is broken by higher-order perturbation terms. This enforces finite but parametrically small gaps at some low-symmetry points in momentum space. Untethered from the restraints of symmetry, quasi-symmetries eliminate the need for fine tuning as they enforce that sources of large Berry curvature occur at arbitrary chemical potentials. We demonstrate that quasi-symmetry in the semi-metal CoSi stabilizes gaps below 2 meV over a large near-degenerate plane that can be measured in the quantum oscillation spectrum. The application of in-plane strain breaks the crystal symmetry and gaps the degenerate point, observable by new magnetic breakdown orbits. The quasi-symmetry, however, does not depend on spatial symmetries and hence transmission remains fully coherent. These results demonstrate a class of topological materials with increased resilience to perturbations such as strain-induced crystalline symmetry breaking, which may lead to robust topological applications as well as unexpected topology beyond the usual space group classifications.The concept of quasi-symmetry—a perturbatively small deviation from exact symmetry—is introduced and leads to topological materials with strong resilience to perturbations.
Bending strain in 3D topological semi-metals Diaz, Jonas; Putzke, Carsten; Huang, Xiangwei ...
Journal of physics. D, Applied physics,
02/2022, Letnik:
55, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Abstract
We present an experimental set-up for the controlled application of strain gradients by mechanical piezoactuation on 3D crystalline microcantilevers that were fabricated by focused ion beam ...machining. A simple sample design tailored for transport characterization under strain at cryogenic temperatures is proposed. The topological semi-metal Cd
3
As
2
serves as a test bed for the method, and we report extreme strain gradients of up to
1.3
%
μ
m
−1
at a surface strain value of
≈
0.65
%
at 4 K. Interestingly, the unchanged quantum transport of the cantilever suggests that the bending cycle does not induce defects via plastic deformation. This approach is a first step towards realizing transport phenomena based on structural gradients, such as artificial gauge fields in topological materials.
As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm's law. Depending on the length scales of momentum conserving (l
) and relaxing (l
...) electron scattering, and the device size (d), current flows may shift from ohmic to ballistic to hydrodynamic regimes. So far, an in situ methodology to obtain these parameters within a micro/nanodevice is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain l
even when l
≫ d. We extract l
from the Sondheimer amplitude in WP
, at temperatures up to T ~ 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on μm-sized devices are in excellent agreement with experimental reports of the bulk l
and confirm that WP
can be microfabricated without degradation. These results conclusively establish Sondheimer oscillations as a quantitative probe of l
in micro-devices.
While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low ...carrier density semi-metals, such as long-ranged current jets forming along the magnetic field when currents pass through point-like constrictions. Occurring accidentally at imperfect current injection contacts, the phenomenon of "current jetting" plagues the research of longitudinal magneto-resistance, which is particularly important in topological conductors. Here we demonstrate the controlled generation of tightly focused electron beams in a new class of micro-devices machined from crystals of the Dirac semi-metal Cd3As2. The current beams can be guided by tilting a magnetic field and their range tuned by the field strength. Finite element simulations quantitatively capture the voltage induced at faraway contacts when the beams are steered towards them, supporting the picture of controlled electron jets. These experiments demonstrate direct control over the highly non-local signal propagation unique to 3D semi-metals in the current jetting regime, and may lead to applications akin to electron optics in free space.
The crystal symmetry of a material dictates the type of topological band structures it may host, and therefore symmetry is the guiding principle to find topological materials. Here we introduce an ...alternative guiding principle, which we call 'quasi-symmetry'. This is the situation where a Hamiltonian has an exact symmetry at lower-order that is broken by higher-order perturbation terms. This enforces finite but parametrically small gaps at some low-symmetry points in momentum space. Untethered from the restraints of symmetry, quasi-symmetries eliminate the need for fine-tuning as they enforce that sources of large Berry curvature will occur at arbitrary chemical potentials. We demonstrate that a quasi-symmetry in the semi-metal CoSi stabilizes gaps below 2 meV over a large near-degenerate plane that can be measured in the quantum oscillation spectrum. The application of in-plane strain breaks the crystal symmetry and gaps the degenerate point, observable by new magnetic breakdown orbits. The quasi-symmetry, however, does not depend on spatial symmetries and hence transmission remains fully coherent. These results demonstrate a class of topological materials with increased resilience to perturbations such as strain-induced crystalline symmetry breaking, which may lead to robust topological applications as well as unexpected topology beyond the usual space group classifications.
As conductors in electronic applications shrink, microscopic conduction
processes lead to strong deviations from Ohm's law. Depending on the length
scales of momentum conserving ($l_{MC}$) and ...relaxing ($l_{MR}$) electron
scattering, and the device size ($d$), current flows may shift from ohmic to
ballistic to hydrodynamic regimes and more exotic mixtures thereof. So far, an
in situ, in-operando methodology to obtain these parameters self-consistently
within a micro/nanodevice, and thereby identify its conduction regime, is
critically lacking. In this context, we exploit Sondheimer oscillations,
semi-classical magnetoresistance oscillations due to helical electronic motion,
as a method to obtain $l_{MR}$ in micro-devices even when $l_{MR}\gg d$. This
gives information on the bulk $l_{MR}$ complementary to quantum oscillations,
which are sensitive to all scattering processes. We extract $l_{MR}$ from the
Sondheimer amplitude in the topological semi-metal WP$_2$, at elevated
temperatures up to $T\sim 50$~K, in a range most relevant for hydrodynamic
transport phenomena. Our data on micrometer-sized devices are in excellent
agreement with experimental reports of the large bulk $l_{MR}$ and thus confirm
that WP$_2$ can be microfabricated without degradation. Indeed, the measured
scattering rates match well with those of theoretically predicted
electron-phonon scattering, thus supporting the notion of strong momentum
exchange between electrons and phonons in WP$_2$ at these temperatures. These
results conclusively establish Sondheimer oscillations as a quantitative probe
of $l_{MR}$ in micro-devices in studying non-ohmic electron flow.
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