One of the most fundamental devices for electronics and optoelectronics is the PN junction, which provides the functional element of diodes, bipolar transistors, photodetectors, LEDs, and solar cells, among many other devices. In conventional PN junctions, the adjacent p-and ntype regions of a semiconductor are formed by chemical doping. Materials with ambipolar conductance, however, allow for PN junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here we report ambipolar monolayer WSe 2 devices in which two local gates are used to define a PN junction exclusively within the sheet of WSe 2 . With these electrically tunable PN junctions, we demonstrate both PN and NP diodes with ideality factors better than 2. Under excitation with light, the diodes show photodetection responsivity of 210 mA/W and photovoltaic power generation with a peak external quantum efficiency of 0.2%, promising numbers for a nearly transparent monolayer sheet in a lateral device geometry. Finally, we demonstrate a lightemitting diode based on monolayer WSe 2 . These devices provide a fundamental building block for ubiquitous, ultra-thin, flexible, and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials.Next generation photodetectors and photovoltaic devices, as well as sensors, displays, and light emitting diodes, will all require new optoelectronic materials with superior characteristics to those currently in use. Candidate materials must be flexible for wearable devices, transparent for interactive displays, efficient for solar cells, and robust and low cost for broad distribution. Monolayer semiconducting transition metal dichalcogenides (TMDs) such as tungsten diselenide (WSe 2 ) are flexible 1 , nearly transparent 2,3 , high strength 1 , direct band gap 2 materials that have the potential to meet all of these criteria. Tungsten diselenide's crystal structure is comprised of stacks of trilayer sheets made of a single atomic layer of tungsten encapsulated by two layers of selenium. This structure leads to very strong intralayer bonding in the trilayer plane and weak interlayer bonding * These authors contributed equally to this work.arXiv:1310.0452v2 [cond-mat.mes-hall]

We report electronic transport measurements of devices based on monolayers and bilayers of the transition-metal dichalcogenide MoS 2 . Through a combination of in situ vacuum annealing and electrostatic gating we obtained ohmic contact to the MoS 2 down to 4 K at high carrier densities. At lower carrier densities, low temperature four probe transport measurements show a metal-insulator transition in both monolayer and bilayer samples. In the metallic regime, the high temperature behavior of the mobility showed strong temperature dependence consistent with phonon dominated transport. At low temperature, intrinsic field-effect mobilities greater than 1000 cm 2 /Vs were observed for both monolayer and bilayer devices. Mobilities extracted from Hall effect measurements were several times lower and showed a strong dependence on density, likely caused by screening of charged impurity scattering at higher densities.

When electrons in a solid are excited with light, they can alter the free energy landscape and access phases of matter that are beyond reach in thermal equilibrium. This accessibility becomes of vast importance in the presence of phase competition, when one state of matter is preferred over another by only a small energy scale that, in principle, is surmountable by light. Here, we study a layered compound, LaTe3, where a small in-plane (a-c plane) lattice anisotropy results in a unidirectional charge density wave (CDW) along the c-axis. Using ultrafast electron diffraction, we find that after photoexcitation, the CDW along the c-axis is weakened and subsequently, a different competing CDW along the a-axis emerges. The timescales characterizing the relaxation of this new CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. The new density wave represents a transient non-equilibrium phase of matter with no equilibrium counterpart, and this study thus provides a framework for unleashing similar states of matter that are "trapped" under equilibrium conditions.

Layered transition metal dichalcogenides that host coexisting charge-density wave (CDW) and superconducting orders provide ideal systems for exploring the effects of dimensionality on correlated electronic phases. Dimensionality has a profound effect on both superconductivity and CDW instabilities. Here we report a substantial enhancement of the superconducting Tc to 3.4 K for 2H-TaS2 in the monolayer limit, compared to 0.8 K in the bulk. In addition, the transport signature of a CDW phase transition vanishes in the 2D limit. In our analysis of electronic and vibrational properties of this material, we show that a reduction of the CDW amplitude results in a substantial increase of the density of states at the Fermi energy, which can boost Tc by an amount similar to that seen in experiment. Our results indicate competition between CDW order and superconductivity in ultra-thin 2H-TaS2 down to the monolayer limit, providing insight towards understanding correlated electronic phases in reduced dimensions.

We propose an extended version of the classical Karhunen-Loève expansion of a multivariate random process, termed a normalized multivariate functional principal component (mFPCn) representation. This takes variations between the components of the process into account and takes advantage of component dependencies through the pairwise cross-covariance functions. This approach leads to a single set of multivariate functional principal component scores, which serve well as a proxy for multivariate functional data. We derive the consistency properties for the estimates of the mFPCn, and the asymptotic distributions for statistical inferences. We illustrate the finite sample performance of this approach through the analysis of a traffic flow data set, including an application to clustering and a simulation study. The mFPCn approach serves as a basic and useful statistical tool for multivariate functional data analysis.