134 research outputs found
Long valley lifetime of dark excitons in single-layer WSe2
Single-layer transition metal dichalcogenides (TMDs) provide a promising
material system to explore the electron's valley degree of freedom as a quantum
information carrier. The valley degree of freedom in single-layer TMDs can be
directly accessed by means of optical excitation. The rapid valley relaxation
of optically excited electron-hole pairs (excitons) through the long-range
electron-hole exchange interaction, however, has been a major roadblock.
Theoretically such a valley relaxation does not occur for the recently
discovered dark excitons, suggesting a potential route for long valley
lifetimes. Here we investigate the valley dynamics of dark excitons in
single-layer WSe2 by time-resolved photoluminescence spectroscopy. We develop a
waveguide-based method to enable the detection of the dark exciton emission,
which involves spin-forbidden optical transitions with an out-of-plane dipole
moment. The valley degree of freedom of dark excitons is accessed through the
valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a
short valley lifetime for the dark neutral exciton, likely due to the
short-range electron-hole exchange, but long valley lifetimes exceeding several
nanoseconds for dark charged excitons
Electric-field switching of two-dimensional van der Waals magnets
Controlling magnetism by purely electrical means is a key challenge to better
information technology1. A variety of material systems, including ferromagnetic
(FM) metals2,3,4, FM semiconductors5, multiferroics6,7,8 and magnetoelectric
(ME) materials9,10, have been explored for the electric-field control of
magnetism. The recent discovery of two-dimensional (2D) van der Waals
magnets11,12 has opened a new door for the electrical control of magnetism at
the nanometre scale through a van der Waals heterostructure device platform13.
Here we demonstrate the control of magnetism in bilayer CrI3, an
antiferromagnetic (AFM) semiconductor in its ground state12, by the application
of small gate voltages in field-effect devices and the detection of
magnetization using magnetic circular dichroism (MCD) microscopy. The applied
electric field creates an interlayer potential difference, which results in a
large linear ME effect, whose sign depends on the interlayer AFM order. We also
achieve a complete and reversible electrical switching between the interlayer
AFM and FM states in the vicinity of the interlayer spin-flip transition. The
effect originates from the electric-field dependence of the interlayer exchange
bias.Comment: 12 pages, 4 figure
Experimental demonstration of continuous electronic structure tuning via strain in atomically thin MoS2
We demonstrate the continuous tuning of the electronic structure of
atomically thin MoS2 on flexible substrates by applying a uniaxial tensile
strain. A redshift at a rate of ~70 meV per percent applied strain for direct
gap transitions, and at a rate 1.6 times larger for indirect gap transitions,
have been determined by absorption and photoluminescence spectroscopy. Our
result, in excellent agreement with first principles calculations, demonstrates
the potential of twodimensional crystals for applications in flexible
electronics and optoelectronics.Comment: 19 pages and 4 figures including supporting information. Nano Lett.,
2013, 13 (6), pp 293
Valley-selective exciton bistability in a suspended monolayer semiconductor
We demonstrate robust power- and wavelength-dependent optical bistability in
fully suspended monolayers of WSe2 near the exciton resonance. Bistability has
been achieved under continuous-wave optical excitation at an intensity level of
10^3 W/cm^2. The observed bistability is originated from a photo-thermal
mechanism, which provides both optical nonlinearity and passive feedback, two
essential elements for optical bistability. Under a finite magnetic field, the
exciton bistability becomes helicity dependent, which enables repeatable
switching of light purely by its polarization.Comment: 14 pages, 6 figure
Possible Topological Superconducting Phases of MoS
Molybdenum disulphide (MoS) has attracted much interest in recent years
due to its potential applications in a new generation of electronic devices.
Recently, it was shown that thin films of MoS can become superconducting
with a highest of 10K when the material is heavily gated to the
conducting regime. In this work, using the group theoretical approach, we
determine the possible pairing symmetries of heavily gated MoS. Depending
on the electron-electron interactions, the material can support an exotic
spin-singlet -wave-like, an exotic spin-triplet s-wave-like and an
conventional spin-triplet -wave pairing phases. Importantly, the exotic
spin-singlet -wave phase is a topological superconducting phase which
breaks time-reversal symmetry spontaneously and possesses chiral Majorana edge
states.Comment: 5 pages, 4 figures. References added. Comments are welcom
Controlling magnetism in 2D CrI3 by electrostatic doping
The atomic thickness of two-dimensional (2D) materials provides a unique
opportunity to control material properties and engineer new functionalities by
electrostatic doping. Electrostatic doping has been demonstrated to tune the
electrical and optical properties of 2D materials in a wide range, as well as
to drive the electronic phase transitions. The recent discovery of atomically
thin magnetic insulators has opened up the prospect of electrical control of
magnetism and new devices with unprecedented performance. Here we demonstrate
control of the magnetic properties of monolayer and bilayer CrI3 by
electrostatic doping using a dual-gate field-effect device structure. In
monolayer CrI3, doping significantly modifies the saturation magnetization,
coercive force and Curie temperature, showing strengthened (weakened) magnetic
order with hole (electron) doping. Remarkably, in bilayer CrI3 doping
drastically changes the interlayer magnetic order, causing a transition from an
antiferromagnetic ground state in the pristine form to a ferromagnetic ground
state above a critical electron density. The result reveals a strongly
doping-dependent interlayer exchange coupling, which enables robust switching
of magnetization in bilayer CrI3 by small gate voltages.Comment: 12 pages and 4 figure
Observation of the nonlinear anomalous Hall effect in 2D WTe2
The Hall effect occurs only in systems with broken time-reversal symmetry,
such as solids under an external magnetic field in the ordinary Hall effect and
magnetic materials in the anomalous Hall effect (AHE). Here we show a new Hall
effect in a nonmagnetic material under zero magnetic field, in which the Hall
voltage depends quadratically on the longitudinal current. We observe the
effect (referred to as nonlinear AHE) in two-dimensional Td-WTe2, a semimetal
with broken inversion symmetry and only one mirror line in the crystal plane.
Our angle-resolved electrical measurements reveal that the Hall voltage changes
sign when the bias current reverses direction; it maximizes (vanishes) when the
bias current is perpendicular (parallel) to the mirror line. The observed
effect can be understood as an AHE induced by the bias current which generates
an out-of-plane magnetization. The temperature dependence of the Hall
conductivity further suggests that both intrinsic Berry curvature dipole and
extrinsic spin-dependent scatterings contribute to the observed nonlinear AHE.
Our results open the possibility of exploring the intrinsic Berry curvature
effect in nonlinear electrical transport in solids
The Electronic Structure of Few-Layer Graphene: Probing the Evolution from a 2-Dimensional to a 3-Dimensional Material
While preserving many of the unusual features of single-layer graphene,
few-layer graphene (FLG) provides a richness and flexibility of electronic
structure that render this set of materials of great interest for both
fundamental studies and applications. Essential for progress, however, is an
understanding of the evolution of the electronic structure of these materials
with increasing layer number. In this report, the evolution of the electronic
structure of FLG, for N = 1 - 8 atomic layers, has been characterized by
measurements of the optical conductivity spectra. For each layer thickness N,
distinctive peaks are found in the infrared range, with positions obeying a
simple scaling relation. The observations are explained by a unified
zone-folding scheme that generates the electronic structure for all FLG
materials from that of bulk graphite.Comment: 15 pages, 5 figure
Two-dimensional magnetic nanoelectromechanical resonators
Two-dimensional (2D) layered materials possess outstanding mechanical,
electronic and optical properties, making them ideal materials for
nanoelectromechanical applications. The recent discovery of 2D magnetic
materials has promised a new class of magnetically active nanoelectromechanical
systems (NEMS). Here we demonstrate resonators made of 2D CrI3, whose
mechanical resonances depend on the magnetic state of the material. We quantify
the underlining effects of exchange and anisotropy magnetostriction by
measuring the field dependence of the resonance frequency under a magnetic
field parallel and perpendicular to the easy axis, respectively. Furthermore,
we show efficient strain tuning of magnetism in 2D CrI3 as a result of the
inverse magnetostrictive effect using the NEMS platform. Our results establish
the basis for mechanical detection of magnetism and magnetic phase transitions
in 2D layered magnetic materials. The new magnetic NEMS may also find
applications in magnetic actuation and sensing
Thermal conductance at the graphene-SiO2 interface measured by optical pump-probe spectroscopy
We have examined the interfacial thermal conductance {\sigma}int of single
and multi-layer graphene samples prepared on fused SiO2 substrates by
mechanical exfoliation of graphite. By using an ultrafast optical pump pulse
and monitoring the transient reflectivity on the picosecond time scale, we
obtained an average {\sigma}int of 5,000 W/cm2K for the graphene-SiO2 system.
We observed significant variation in {\sigma}int between individual samples,
but found no systematic dependence on the thickness of the graphene layers.Comment: 13 pages, 3 figure
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