81 research outputs found
Multivalued current-phase relationship in a.c. Josephson effect for a three-dimensional Weyl semimetal WTe
We experimentally study electron transport between two superconducting indium
leads, coupled to a single WTe crystal, which is a three-dimensional Weyl
semimetal. We demonstrate Josephson current in long 5~m In-WTe-In
junctions, as confirmed by the observation of integer (1,2,3) and fractional
(1/3, 1/2, 2/3) Shapiro steps under microwave irradiation. Demonstration of
fractional a.c. Josephson effect indicates multivalued character of the
current-phase relationship, which we connect with Weyl topological surface
states contribution to Josephson current. In contrast to topological insulators
and Dirac semimetals, we do not observe periodicity in a.c. Josephson
effect for WTe at different frequencies and power, which might reflect
chiral character of the Fermi arc surface states in Weyl semimetal.Comment: the text is seriously corrected. arXiv admin note: text overlap with
arXiv:1801.0955
Signature of Fermi arc surface states in Andreev reflection at the WTe Weyl semimetal surface
We experimentally investigate charge transport through the interface between
a niobium superconductor and a three-dimensional WTe Weyl semimetal. In
addition to classical Andreev reflection, we observe sharp non-periodic subgap
resistance resonances. From an analysis of their positions, magnetic field and
temperature dependencies, we can interpret them as an analog of Tomasch
oscillations for transport along the topological surface state across the
region of proximity-induced superconductivity at the Nb-WTe interface.
Observation of distinct geometrical resonances implies a specific transmission
direction for carriers, which is a hallmark of the Fermi arc surface states.Comment: 5 pages, some misprints has been correcte
Multiple magnon modes in the CoSnS Weyl semimetal candidate
We experimentally investigate electron transport in kagome-lattice
ferromagnet CoSnS, which is regarded as a time-reversal symmetry
broken Weyl semimetal candidate. We demonstrate curves with
pronounced asymmetric spikes, similar to those attributed to
current-induced spin-wave excitations in ferromagnetic multilayers. In contrast
to multilayers, we observe several spikes' sequences at low,
10 A/cm, current densities for a thick single-crystal
CoSnS flake in the regime of fully spin-polarized bulk. The spikes
at low current densities can be attributed to novel magnon branches in magnetic
Weyl semimetals, which are predicted due to the coupling between two magnetic
moments mediated by Weyl fermions. Presence of spin-transfer effects at low
current densities in CoSnS makes the material attractive for
applications in spintronics.Comment: final versio
Incorporating model uncertainty into optimal insurance contract design
In stochastic optimization models, the optimal solution heavily depends on the selected probability model for the scenarios. However, the scenario models are typically chosen on the basis of statistical estimates and are therefore subject to model error. We demonstrate here how the model uncertainty can be incorporated into the decision making process. We use a nonparametric approach for quantifying the model uncertainty and a minimax setup to find model-robust solutions. The method is illustrated by a risk management problem involving the optimal design of an insurance contract
Catalogue and Toolbox of Risk Assessment and Management Tools
The ENHANCE project is concerned with analysing and working towards improved public-private partnerships for managing risks from natural hazards. An important issue for such partnerships is the methods, tools and processes available for assessing risk and risk management options. Risk analysis has long provided useful input to decision-making. At the same time, the field of risk analysis is in motion and an enhanced framing of risk analysis and risk management is being embraced following an iterative cycle organized around notions of learning, innovation and transformation. This broadened vision on risk analysis is a key issue for the ENHANCE project as well, which takes many and different perspectives on analysing, understanding, communicating and managing risk.
This report lays out the status quo at the outset of the project regarding risk analytical tools, methods and data that are currently used by project partners in ENHANCE. The task overall develops a catalogue of existing risk assessment and management tools and methods to describe the concepts of iterative risk management and further sets up a toolbox, containing individual models and tools to be used by the case studies in their analyses.
While work in the cases study, including methodological development, is in process, we find that ENHANCE partners and cases employ a multitude of models, tools and data ranging from impact analysis, different risk modelling techniques to various decision-support methods. A number of tools that encapsulate these methods are also available with the consortium.
We suggest the tools and methods in use can be useful starting points for working towards a broader vision of iterative risk management. While the work so far, and this deliverable, have focussed on populating the technical stages of the risk analytical cycle (visually identified as the inner circle), we suggest in the next phase of ENHANCE, additional efforts should be dispensed to better understand adaptive management aspects associated with using these methods and tools, such as learning, innovation and transformation, which we exhibit visually in an outer circle.
This report proceeds as follows: We start with laying out key elements of risk analysis and management in section 2, which also describes the new framing organized around the iterative risk-management concept. Methods for assessing risk and evaluating risk management are discussed in section 3. Then we consider methods, models and datasets that are in use in the ENHANCE case studies at the moment (section 4), before section 5 concludes. Finally and importantly, the annex lists more information on cases studies, for which detailed information was received from the project partners
Surface ferromagnetism in a chiral topological semimetal CoSi
Despite the chiral topological semimetal CoSi is known as bulk diamagnetic,
it shows unusual surface ferromagnetism of debatable origin. The ferromagnetic
ordering has been attributed to the distorted bonds, the superlattice of
ordered vacancies, or even to topological surface textures due to the spin
polarization in the neighboring Fermi arcs. We experimentally compare
magnetization reversal curves for initially oxidized CoSi single crystals and
cleaved samples with a fresh, oxide-free surface. While the oxidized CoSi
samples do not show sizable ferromagnetism, the fresh CoSi surface gives a
strong ferromagnetic response, which is accompanied by the pronounced
modulation of the angle dependence of magnetization, as it can be expected for
easy and hard axes in a ferromagnet. In addition to the first order reversal
curves analysis, this observation allows us to distinguish between different
mechanisms of the ferromagnetic ordering in CoSi single crystals. We conclude
that the surface states-induced RKKY interaction between distorted bonds near
the sample surface is responsible for the strong ferromagnetic multi-domain
behavior for freshly cleaved samples
Surface spin polarization in the magnetic response of GeTe Rashba ferroelectric
We experimentally investigate magnetization reversal curves for a GeTe
topological semimetal. In addition to the known lattice diamagnetic response,
we observe narrow magnetization loop in low fields, which should not be
expected for non-magnetic GeTe. The hysteresis is unusual, so the saturation
level is negative in positive fields, and the loop is passed clockwise, in
contrast to standard ferromagnetic behavior. The experimental hysteresis curves
can not be obtained from usual ferromagnetic ones by adding/subtracting of any
linear dependence, or even by considering several interacting magnetic phases.
The possibility of several phases is also eliminated by the remanence plots
technique (Henkel or {\delta}M plots). We explain our results as a direct
consequence of the correlation between ferroelectricity and spin-polarized
surface states in GeTe, similarly to magnetoelectric structures
Switching ferroelectricity in SnSe across diffusionless martensitic phase transition
We experimentally investigate transport properties of a hybrid structure,
which consists of a thin single crystal SnSe flake on a top of 5~m spaced
Au leads. The structure initially is in highly-conductive state, while it can
be switched to low-conductive one at high currents due to the Joule heating of
the sample, which should be identified as - -- -
diffusionless martensitic phase transition in SnSe. For highly-conductive
state, there is significant hysteresis in curves at low biases, so
the sample conductance depends on the sign of the applied bias change. This
hysteretic behavior reflects slow relaxation due to additional polarization
current in the ferroelectric SnSe phase, which we confirm by direct measurement
of time-dependent relaxation curves. In contrast, we observe no noticeable
relaxation or low-bias hysteresis for the quenched -
low-conductive phase. Thus, ferroelectric behavior can be switched on or off in
transport through hybrid SnSe structure by controllable - --
- phase transition. This result can also be important for
nonvolatile memory development, e.g. phase change memory for neuromorphic
computations or other applications in artificial intelligence and modern
electronics
- …