The ability to detect light over a broad spectral range is central for practical optoelectronic applications, and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS 2 . However, polarization sensitivity within such a photodetector remains elusive. Here we demonstrate a linear-dichroic broadband photodetector with layered black phosphorus transistors, using the strong intrinsic linear dichroism arising from the in-plane optical anisotropy with respect to the atom-buckled direction, which is polarization sensitive over a broad bandwidth from about 400 nm to 3750 nm. Especially, a perpendicular built-in electric field induced by gating in the transistor geometry can spatially separate the photo-generated electrons and holes in the channel, effectively reducing their recombination rate, and thus enhancing the performance for linear dichroism photodetection. This provides practical functionality using anisotropic layered black phosphorus, thereby enabling novel optical and optoelectronic device applications. Corresponding author: hyhwang@stanford.edu, yicui@stanford.edu. 2Confined electronic systems in layered two-dimensional (2D) crystals are host to many emerging electronic, spintronic and photonic phenomena, 1, 2, 3 including quantum Hall and Dirac electrons in graphene 4, 5, 6 and topological surface states in topological insulators 7, 8 . Experimentally identifying new functionalities of two-dimensional materials is a challenging and rewarding frontier, enabled by recent advances in materials and device fabrication. One example is the valley polarization control using circularly polarized light in the non-centrosymmetric MoS 2 monolayer and resulting potential valleytronics applications. 9, 10,11 Other examples include recent demonstrations of novel electronic and optoelectronic applications of the well-known layered material black phosphorus (BP), such as high-mobility field effect transistors and linear-polarization dependent optical absorption. 12,13,14 Therefore, further discovering new properties and functionalities utilizing known layered materials is of practical importance and great current interest. 14,15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 As a potential functionality for layered 2D materials, linear dichroism (LD) is an electromagnetic spectroscopy probing different absorption of light polarized parallel or perpendicular to an orientation axis. It directly depends on the conformation and orientation of material/device structures, where they are either intrinsically oriented in an anisotropic crystal structure 27, 28 or extrinsically oriented in anisotropic device patterns 29, 30 . Compared to the hexagonal in-plane lattice in other 2D materials such as graphene and MoS 2 , which are insensitive to the linear polarization of incident light, the layered BP crystal with a rectangular in-plane lattice has a highly-anisotropic structure along the x and y directions (defined in Fig. 1a), where every two rows of P atoms alternatel...

LaAlO 3 and SrTiO 3 are insulating, non-magnetic oxides, yet the interface between them exhibits a two-dimensional electron system with high electron mobility 1 , superconductivity at low temperatures 2-6 and electric-field-tuned metal-insulator and superconductor-insulator phase transitions 3,6-8. Bulk magnetization and magnetoresistance measurements also indicate some form of magnetism depending on preparation conditions 5,9-11 and a tendency towards nanoscale electronic phase separation 10. Here we use local imaging of the magnetization and magnetic susceptibility to directly observe a landscape of ferromagnetism, paramagnetism and superconductivity. We find submicrometre patches of ferromagnetism in a uniform background of paramagnetism, with a non-uniform, weak diamagnetic superconducting susceptibility at low temperature. These results demonstrate the existence of nanoscale phase separation as indicated by theoretical predictions based on nearly degenerate interface sub-bands associated with the Ti orbitals 12,13. The magnitude and temperature dependence of the paramagnetic response indicate that the vast majority of the electrons at the interface are localized 14 , and do not contribute to transport measurements 3,6,7. In addition to the implications for magnetism, the existence of a two-dimensional superconductor at an interface with highly broken inversion symmetry and a ferromagnetic landscape in the background indicates the potential for exotic superconducting phenomena. Coexistence of ferromagnetism and superconductivity in nature is rare 15-19. The LaAlO 3 /SrTiO 3 (LAO/STO) interface is a new system for studying this coexistence. LAO and STO are both perovskite band insulators with no magnetic order in their bulk form. For LAO grown on the TiO 2-terminated STO substrate, a high-mobility electron gas was observed at the interface 1. Electronic reconstruction, driven by the polar/nonpolar interface, is thought to move charge from the LAO layers across the interface into the STO, causing an effective electronic doping responsible for the observed conductivity 1. The interplay of this effect with oxygen vacancies and structural changes 20 , and the relative contribution of these three effects to the carrier concentration, remains a subject of debate. Significant variability in the physical properties in similar samples indicates that the ground state of this interface system is sensitive to small changes in growth conditions. Superconductivity 2-5 and features interpreted as interface magnetism 5,9,10 have been independently observed at the LAO/STO interface through transport and bulk magnetization measurements. One recent study inferred the existence of both LETTERS NATURE PHYSICS

Caviglia et al. [Nature (London) 456, 624 (2008)] have found that the superconducting LaAlO3/SrTiO3 interface can be gate modulated. A central issue is to determine the principal effect of the applied electric field. Using magnetotransport studies of a gated structure, we find that the mobility variation is almost five times as large as the sheet carrier density. Furthermore, superconductivity can be suppressed at both positive and negative gate bias. These results indicate that the relative disorder strength strongly increases across the superconductor-insulator transition.PACS numbers: 74.78.Db, 85.30.Tv The strength of the electric field effect (EFE) in accumulating or depleting carriers in a conducting channel is central not only to many semiconductor devices found ubiquituously in modern electronics, but also in current research into achieving novel physics using tunable materials. Complex oxides are one case where the electronic ground state of the system is highly sensitive to the carrier density [1,2]. Among the commonly studied oxide materials, SrTiO 3 has attracted much attention due to its high electron mobility and electric permittivity at low temperatures, which facilitates large electric field effects [3]. Many recent oxide EFE devices have utilized SrTiO 3 substrates as a crucial component of the experiment: both the metallicity and superconductivity of SrTiO 3 have been modulated [2,4,5,6].Recently Caviglia et al.[2] strikingly demonstrated that the EFE could be used to modulate the superconductivity which appears [7] in the metallic gas formed between the two insulators LaAlO 3 and SrTiO 3 . Since its first discovery [8] the origin and physics of this metallic layer has been intensively investigated. Room temperature scanning electron energy-loss spectroscopy and conducting scanning probe measurements have set an upper limit of ∼7 nm for the gas thickness in annealed or high pressure grown samples [9,10]. However it is still unclear how the gas thickness is correlated to the sheet resistance at low temperatures, which itself can be changed by several orders of magnitude with oxygen pressure during LaAlO 3 growth [8,11], or the EFE [2,5].In this context, Caviglia et al. assumed that the superconductivity suppression by the EFE is due to the reduction of the carrier density of the electron gas. However it was unclear how the mobility of the electron gas changed in these initial EFE experiments, since it was not probed. A direct measurement of the mobility is vital in order to experimentally determine what is the effective tuning parameter of the superconductor-insulator transition. In this Letter, we describe a detailed study of the magnetotransport properties of a LaAlO 3 /SrTiO 3 interface, for which superconductivity can be fully suppressed by both positive and negative gate bias. Normal state magnetotransport measurements were also made above the upper critical field at which superconductivity is destroyed. From these data we find that not only does the carrier density vary with applied ga...

A number of recent transport and magnetization studies have shown signs of ferromagnetism in the LaAlO 3 /SrTiO 3 heterostructure 1-6 , an unexpected property with no bulk analog in the constituent materials. However, no experiment thus far has provided direct information on the host of the magnetism 7-11 . Here we report spectroscopic investigations of the magnetism using element-specific techniques, including x-ray magnetic circular dichroism and x-ray absorption spectroscopy, along with corresponding model calculations. We find direct evidence for in-plane ferromagnetic order at the interface, with Ti 3+ character in the d xy orbital of the anisotropic t 2g band. These findings establish a striking example of emergent phenomena at oxide interfaces.Recent advances in the atomic-scale synthesis and characterization of perovskite oxide heterostructures have engendered significant interest in their electronic and magnetic structure. SLAC-PUB-15439Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515 and BES.Given their vast physical properties in bulk form, and their epitaxial compatibility, perovskites provide an ideal arena to explore the competition, interaction, and creation of many ground states at their interfaces 12 . The LaAlO 3 /SrTiO 3 heterostructure is a canonical example, exhibiting interface conductivity 13 , superconductivity 14 , and ferromagnetism 1-6 at the interface between two wide band-gap insulators. From a fundamental perspective, ferromagnetism is perhaps the most important property; although bulk SrTiO 3 can be doped to be metallic and superconducting, neither constituent in bulk form exhibits ferromagnetism. Hence interface ferromagnetism here could be a leading example of truly emergent phenomena. Most previous studies used bulk probes (macroscopic magnetization or torque) 3, 4 ; while scanning SQUID microscopy could localize the magnetism to the near surface region 5, 15 , the specific location where the moments reside is beyond the resolution of the probe. In principle, magnetism could arise from cation/anion defects in the LaAlO 3 or SrTiO 3 , or could be specific to the interface; theoretical scenarios have been proposed for all of these mechanisms 7-11 . Thus it is of central importance to determine the microscopic nature of the observed ferromagnetism.To address this issue, we applied element-specific techniques at the LaAlO 3 /SrTiO 3 (001) interface, namely synchrotron radiation based x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) (see Materials and Methods section). These measurements can uniquely determine whether the observed magnetization is due to a magnetic moment ( ) from one of the constituent elements, or from extrinsic impurities. All spectra were acquired by recording the total electron yield (TEY). Since the maximum probing depth of TEY is approximately 5~10 nm, these measurements are very sensitive to the interface with proper choice of LaAlO 3 thickness. Using the angle dependence of the XMCD signal, whic...