63 research outputs found
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
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
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
Gate-dependent non-linear Hall effect at room temperature in topological semimetal GeTe
We experimentally investigate non-linear Hall effect as zero-frequency and
second-harmonic transverse voltage responses to ac electric current for
topological semimetal GeTe. A thick single-crystal GeTe flake is placed on the
Si/SiO substrate, where the p-doped Si layer serves as a gate electrode. We
confirm, that electron concentration is not gate-sensitive in thick GeTe flakes
due to the gate field screening by bulk carriers. In contrast, by transverse
voltage measurements, we demonstrate that the non-linear Hall effect shows
pronounced dependence on the gate electric field at room temperature. Since the
non-linear Hall effect is a direct consequence of a Berry curvature dipole in
topological media, our observations indicate that Berry curvature can be
controlled by the gate electric field. This experimental observation can be
understood as a result of the known dependence of giant Rashba splitting on the
external electric field in GeTe. For possible applications, the zero-frequency
gate-controlled non-linear Hall effect can be used for the efficient broad-band
rectification
Current-induced control of the polarization state in a polar metal based heterostructure SnSe/WTe
The concept of a polar metal proposes new approach of current-induced
polarization control for ferroelectrics. We fabricate SnSe/WTe
heterostructure to experimentally investigate charge transport between two
ferroelectric van der Waals materials with different polarization directions.
WTe is a polar metal with out-of-plane ferroelectric polarization, while
SnSe ferroelectric semiconductor is polarized in-plane, so one should expect
complicated polarization structure at the SnSe/WTe interface. We study
curves, which demonstrate sharp symmetric drop to zero
differential conductance at some threshold bias voltages , which
are nearly symmetric in respect to the bias sign. While the gate electric field
is too small to noticeably affect the carrier concentration, the positive and
negative threshold positions are sensitive to the gate voltage. Also,
SnSe/WTe heterostructure shows re-entrant transition to the low-conductive
state for abrupt change of the bias voltage even below the threshold
values. This behavior can not be observed for single SnSe or WTe flakes, so
we interpret it as a result of the SnSe/WTe interface coupling. In this
case, some threshold value of the electric field at the SnSe/WTe interface
is enough to drive 90 change of the initial SnSe in-plane polarization
in the overlap region. The polarization mismatch leads to the significant
interface resistance contribution, analogously to the scattering of the charge
carriers on the domain walls. Thus, we demonstrate polarization state control
by electron transport through the SnSe/WTe interface
Magnetically stable zero-bias anomaly in Andreev contact to the magnetic Weyl semimetal CoSnS
Being encouraged by the interplay between topology, superconductivity and
magnetism, we experimentally investigate charge transport through the interface
between the Nb superconductor and the time-reversal symmetry breaking Weyl
semimetal CoSnS. In addition to the proximity induced
superconducting gap, we observe several subgap features, among which the most
interesting is the prominent subgap zero-bias anomaly, absolutely stable
against external magnetic fields up to the critical field of Nb. As the
promising scenario for the zero-bias anomaly to appear in transport
characteristics, we consider the proximity induced zero-energy Andreev bound
states interfaced with the half-metallic CoSnS and influenced by
the strong spin-orbit coupling and large Zeeman splitting
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
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