39 research outputs found
Quantum tunneling devices incorporating two-dimensional magnetic semiconductors
Research in two-dimensional (2D) materials has experienced rapid growth in
the past few years. In particular, various layered compounds exhibiting quantum
phenomena, such as superconductivity and magnetism, have been isolated in
atomically thin form, often in spite of their chemical instability. The nature
of the 2D phases can be different than their bulk counterparts, making such
systems attractive for fundamental studies. Owing to their high crystallinity
and absence of dangling bonds, devices and heterostructures incorporating these
materials may also show performance exceeding that of traditional films. In
this roadmap article, we focus on a few recent developments in spin-based
quantum devices utilizing the 2D magnetic semiconductor, CrI.Comment: Invited Review Article for Nanotechnology Roadmap on Quantum
Technolog
Strain Solitons and Topological Defects in Bilayer Graphene
Spontaneous symmetry-breaking, where the ground state of a system has lower
symmetry than the underlying Hamiltonian, is ubiquitous in physics. It leads to
multiply-degenerate ground states, each with a different "broken" symmetry
labeled by an order parameter. The variation of this order parameter in space
leads to soliton-like features at the boundaries of different broken-symmetry
regions and also to topological point defects. Bilayer graphene is a
fascinating realization of this physics, with an order parameter given by its
interlayer stacking coordinate. Bilayer graphene has been a subject of intense
study because in the presence of a perpendicular electric field, a band gap
appears in its electronic spectrum [1-3] through a mechanism that is intimately
tied to its broken symmetry. Theorists have further proposed that novel
electronic states exist at the boundaries between broken-symmetry stacking
domains [4-5]. However, very little is known about the structural properties of
these boundaries. Here we use electron microscopy to measure with nanoscale and
atomic resolution the widths, motion, and topological structure of soliton
boundaries and topological defects in bilayer graphene. We find that each
soliton consists of an atomic-scale registry shift between the two graphene
layers occurring over 6-11 nm. We infer the minimal energy barrier to
interlayer translation and observe soliton motion during in-situ heating above
1000 {\deg}C. The abundance of these structures across a variety samples, as
well as their unusual properties, suggests that they will have substantial
effects on the electronic and mechanical properties of bilayer graphene
Dimensionality-driven orthorhombic MoTe2 at room temperature
We use a combination of Raman spectroscopy and transport measurements to
study thin flakes of the type-II Weyl semimetal candidate MoTe2 protected from
oxidation. In contrast to bulk crystals, which undergo a phase transition from
monoclinic to the inversion symmetry breaking, orthorhombic phase below ~250 K,
we find that in moderately thin samples below ~12 nm, a single orthorhombic
phase exists up to and beyond room temperature. This could be due to the effect
of c-axis confinement, which lowers the energy of an out-of-plane hole band and
stabilizes the orthorhombic structure. Our results suggest that Weyl nodes,
predicated upon inversion symmetry breaking, may be observed in thin MoTe2 at
room temperature
Origin of magnetoresistance suppression in thin -MoTe
We use both classical magnetotransport and quantum oscillation measurements
to study the thickness evolution of the extremely large magnetoresistance (XMR)
material and type-II Weyl semimetal candidate, -MoTe, protected
from oxidation. We find that the magnetoresistance is systematically suppressed
with reduced thickness. This occurs concomitantly with both a decrease in
carrier mobility and increase in electron-hole imbalance. We model the two
effects separately and conclude that the XMR effect is more sensitive to the
former
Tuning Ising superconductivity with layer and spin-orbit coupling in two-dimensional transition-metal dichalcogenides
Systems that simultaneously exhibit superconductivity and spin-orbit coupling
are predicted to provide a route toward topological superconductivity and
unconventional electron pairing, driving significant contemporary interest in
these materials. Monolayer transition-metal dichalcogenide (TMD)
superconductors in particular lack inversion symmetry, enforcing a spin-triplet
component of the superconducting wavefunction that increases with the strength
of spin-orbit coupling. In this work, we present an experimental and
theoretical study of two intrinsic TMD superconductors with large spin-orbit
coupling in the atomic layer limit, metallic 2H-TaS and 2H-NbSe. For
the first time in TaS, we investigate the superconducting properties as the
material is reduced to a monolayer and show that high-field measurements point
to the largest upper critical field thus reported for an intrinsic TMD
superconductor. In few-layer samples, we find that the enhancement of the upper
critical field is sustained by the dominance of spin-orbit coupling over weak
interlayer coupling, providing additional platforms for unconventional
superconducting states in two dimensions.Comment: 8 pages, 4 figures, plus supplemental materia
One million percent tunnel magnetoresistance in a magnetic van der Waals heterostructure
We report the observation of a very large negative magnetoresistance effect
in a van der Waals tunnel junction incorporating a thin magnetic semiconductor,
CrI3, as the active layer. At constant voltage bias, current increases by
nearly one million percent upon application of a 2 Tesla field. The effect
arises from a change between antiparallel to parallel alignment of spins across
the different CrI3 layers. Our results elucidate the nature of the magnetic
state in ultrathin CrI3 and present new opportunities for spintronics based on
two-dimensional materials
Photocurrent Imaging of Multi-Memristive Charge Density Wave Switching in Two-Dimensional 1T-TaS2
Transport studies of atomically thin 1T-TaS2 have demonstrated the presence
of intermediate resistance states across the nearly commensurate (NC) to
commensurate (C) charge density wave (CDW) transition, which can be further
switched electrically. While this presents exciting opportunities for the
material in memristor applications, the switching mechanism has remained
elusive and could be potentially attributed to the formation of inhomogeneous C
and NC domains across the 1T-TaS2 flake. Here, we present simultaneous
electrical driving and scanning photocurrent imaging of CDWs in ultrathin
1T-TaS2 using a vertical heterostructure geometry. While micron-sized CDW
domains form upon changing temperature, electrically driven transitions result
in largely uniform changes, indicating that states of intermediate resistance
for the latter likely correspond to true metastable CDW states in between the
NC and C phases, which we then explain by a free energy analysis. Additionally,
we are able to perform repeatable and bidirectional switching across the
multiple CDW states without changing sample temperature, demonstrating that
atomically thin 1T-TaS2 can be further used as a robust and reversible
multi-memristor material.Comment: This is a preprint of an article accepted for publication in Nano
Letters(2020
Raman fingerprint of two terahertz spin wave branches in a two-dimensional honeycomb Ising ferromagnet
Two-dimensional (2D) magnetism has been long sought-after and only very
recently realized in atomic crystals of magnetic van der Waals materials. So
far, a comprehensive understanding of the magnetic excitations in such 2D
magnets remains missing. Here we report polarized micro-Raman spectroscopy
studies on a 2D honeycomb ferromagnet CrI3. We show the definitive evidence of
two sets of zero-momentum spin waves at frequencies of 2.28 terahertz (THz) and
3.75 THz, respectively, that are three orders of magnitude higher than those of
conventional ferromagnets. By tracking the thickness dependence of both spin
waves, we reveal that both are surface spin waves with lifetimes an order of
magnitude longer than their temporal periods. Our results of two branches of
high-frequency, long-lived surface spin waves in 2D CrI3 demonstrate intriguing
spin dynamics and intricate interplay with fluctuations in the 2D limit, thus
opening up opportunities for ultrafast spintronics incorporating 2D magnets.Comment: 25 pages, 4 figures + 8 supplementary figure
Magneto-memristive switching in a two-dimensional layer antiferromagnet
Memristive devices whose resistance can be hysteretically switched by
electric field or current are intensely pursued both for fundamental interest
as well as potential applications in neuromorphic computing and phase-change
memory. When the underlying material exhibits additional charge or spin order,
the resistive states can be directly coupled, further allowing for electrical
control of the collective phases. Here, we report the observation of abrupt,
memristive switching of tunneling current in nanoscale junctions of ultrathin
CrI, a natural layer antiferromagnet. The coupling to spin order enables
both tuning of the resistance hysteresis by magnetic field, and electric-field
switching of magnetization even in multilayer samples
Generation and detection of coherent longitudinal acoustic waves in ultrathin 1T'-MoTe2
Layered transition metal dichalcogenides have attracted substantial attention
owing to their versatile functionalities and compatibility with current
nanofabrication technologies. Thus, noninvasive means to determine the
mechanical properties of nanometer (nm) thick specimens are of increasing
importance. Here, we report on the detection of coherent longitudinal acoustic
phonon modes generated by impulsive femtosecond (fs) optical excitation.
Broadband fs-transient absorption experiments in 1T'-MoTe2 flakes as a function
of thickness (7 nm - 30 nm) yield a longitudinal sound speed of 2990 m/s. In
addition, temperature dependent measurements unveil a linear decrease of the
normalized Young's modulus with a slope of -0.002 per K and no noticeable
change caused by the Td - 1T' structural phase transition or variations in film
thickness.Comment: The following article has been accepted by Applied Physics Letter